Yes, you can modify the source code according to your requirement. You can directly modify any existing code and add new package into that code.
Once you update the image, do following things to compile the image

goto 'src' directory of tomato distribution.
export the path of the tomato toolchain. If it is in /opt/brcm then give following two commands

export PATH=$toolchainpath/hndtools-mipsel-uclibc/bin:$PATH
export PATH=$toolchainpath/hndtools-mipsel-linux/bin:$PATH

then compile the source code by 'make' command.
If your code successfully compiled then one new image with same name 'tomato_wgr614l_1.25.chk' will be create in the same path. So better take a back up of the old image. Otherwise it will be replaced by your new image.

Are you talking about original firmware of Netgear or Original firmware of tomato 1.25 ?

It is tough to simulate WGR614L with Tomato firmware in a PC because the hardware is totally different. I do not know, but you can find some in the internet where such simulation can be done.
I do not think that changing tomato source code and uploading it into the board would take huge amount of time provided you have serial console for that board. Otherwise you need to upgrade the board from WEB which I would not suggest to you as you said you are not an experience campaigner in this field. If somehow you build a wrong firmware then your board will be gone and it would be difficult (sometimes impossible) for you to recover the board without having any serial console.

It is tough to detail out every thing of the source code. If you have any specific query then I can clarify that one.

Syed Yasir Imtiaz said: Can you suggest me that from where i could read about the serial console ? I dont have any idea about that.. What i understand is that it would be a method by which the firmware could be upgraded using hardware programming. Secondly, kindly tell me is there any guide available to make a backup of the orignal firmware of my router. As you have said that if i could build a wrong firmware then it would be disaster fro me :) ... so in that condition i should at least have a backup of the orignal firmware.

There is a command in the original firmware, called "outputimage" that will take back up. However you can find the original image in the link below

http://www.myopenrouter.com/download/file/10175-1

You get fair enough information from this article about serial console

http://www.myopenrouter.com/article/10341/Recover-Your-WGR614L-Using-a-Serial...

"outputimage" command can be given from the serial console. I forgot that you do not have any serial console.

You will find the original firmware in the link below
http://www.myopenrouter.com/download/10175/WGR614L-Firmware-v1.1.2_1.0.23/

last time total link is not copied. It was my mistake.

Syed Yasir Imtiaz said: Thanks a lot ... its really a helpful information for me :) but the firmware you have mentioned is: firmware update v1.1.2_1.0.23 but my version is v1.1.11_6.0.36 ... If in any case, i would destroy me original firmware i.e v1.1.11_6.0.36 then will it be suitable for me to install the firmware you mentioned ?

The Netgear product support site offers the *.chk image files for both the latest and older official firmware releases for your router.

http://kb.netgear.com/app/products/model/a_id/2590

The *.chk for 1.1.11_6.0.36 is available there. My advice... never, ever, down rev any firmware to releases earlier than the one that's factory installed. Any hidden hardware and firmware rev incompatibility can potentially brick your router. I think what's more important is to backup your router's boot loader and board data, since those are unique to your router, or difficult to find downloads for.

Without a serial console, you can still gain access to the router's command line interface via telnet with the telnet enable utility (for netgear firmware only), and run the outputimage or tftp commands that way. The earlier replies to you about backing up firmware already included those instructions.

Syed Yasir Imtiaz said: Ok ... thats really a useful information for me. As i have told earlier that I am a novice programmer so can you do me a favour please. Kindly tell me all the commands that i have to write using the telnet utility to backup my routers firmware or boot loader and board data. the commands that you posted in the earlier reply do not include the outputimage command therefore kindly tell me all the commands that are neceassary to make a backup. Thanks in advance

The only important router command I needed was tftp. I did not use or look at outputimage at the time. The contributor from  Netgear may be able to comment further on the outputimage. You can also try running command yourself to see if it prints any help or description.

Here's my backup session below. How to restore from these backups is an entirely new project which I haven't looked at.

$
$ # This prompt is Solaris Unix with host IP 192.168.1.10. This host runs a previously configured tftp server.
$
$ # The telnetenable utility for Solaris is available in the download section of http://www.myopenrouter.com/ /> $
$ # Unlock the router's telnet server
$ ./telnetenable 192.168.1.1 001E3A04E2EB Gearguy Geardog
$ # Telnet to the router
$ telnet 192.168.1.1
Trying 192.168.1.1...
Connected to 192.168.1.1.
Escape character is '^]'.


BusyBox v0.60.0 (2008.05.15-10:32+0000) Built-in shell (msh)
Enter 'help' for a list of built-in commands.

# help

Built-in commands:
-------------------
        . : break cd continue eval exec exit export help login newgrp
        read readonly set shift times trap umask wait

# version
Release version : Netgear Wireless Router WGR614v8
                  U12H07200/V1.1.11/6.0.36NA
           Time : May 15 2008 18:35:41
# cd /tmp
# # format of tftp command: tftp <put command> <source file> <destination ip>:<filename>
# # "boot"
# tftp put /dev/mtd/0ro 192.168.1.10:mtd0-cfe
# # "linux"
# tftp put /dev/mtd/1ro 192.168.1.10:mtd1
# # "rootfs"
# tftp put /dev/mtd/2ro 192.168.1.10:mtd2
# # "T_Meter1"
# tftp put /dev/mtd/3ro 192.168.1.10:mtd3
# # "T_Meter2"
# tftp put /dev/mtd/4ro 192.168.1.10:mtd4
# # "POT"
# tftp put /dev/mtd/5ro 192.168.1.10:mtd5
# # "board_data"
# tftp put /dev/mtd/6ro 192.168.1.10:mtd6
# # "nvram"
# tftp put /dev/mtd/7ro 192.168.1.10:mtd7
# exit

Connection to 192.168.1.1 closed by foreign host.
$ # Check that this host's tftp server actually received the router's data
$ ls -la /tftpboot

Is serial console the only way to recover the router? Will telnet still work if the router was corrupted by a wrong modification in the source code?

There are various type of bricking. It is not that telnet will not work, it depends on the level of corruption. But usually it does not work when router is corrupted.
By the way what is your actual problem ? Is your router already corrupted ?

You can choose any firmware among those three as you want. These three source codes are maintained by different community. DD-WRT and tomato are free open source which support other hardware also. On the other hand netgear source code is dedicated for their hardware.
There are lots of difference as far as code is concerned. But I think most important part to know is that Netgear and tomato uses same Linux version (usually it is kernel-2.4.20) whereas DD-WRT uses different Linux version (usually it is kernel-2.4.35)
If you want any specific information let me know.