Have you seen the DD-WRT Wiki page for samba?
--> http://www.dd-wrt.com/wiki/index.php/Samba#Running_a_Samba_Server_on_DD-WRT

I haven't done it myself (and never used ipkg on DD-WRT) but perhaps you should give "Option 3: The easy way" a try. In fact there are even two possibilities in Option 3.
--> http://www.dd-wrt.com/wiki/index.php/Samba#Samba_Server_-_option_3:_the_easy_way

As samba is a Linux tool to share disk space over the network with Windows, I think you have to use a Linux file-system (as ext2 or ext3) - mainly because you want to start samba from this disk which cannot be done from FAT (afaik). You only need these drivers if you want to use the same USB disk directly on a Windows machine.

So the hardest thing you have to do is to format the USB disk or stick with ext3 with a partitioning tool or a Linux Live CD/DVD (like Knoppix --> http://www.knoppix.com/ ).
After that you can just follow the instructions:
http://www.dd-wrt.com/wiki/index.php/Slim_samba2#Install_SlimSamba2_on_a_USB_...

But perhaps someone can help who has done this before?

If samba is not present in DD-WRT then you can add that support. It should not be too much work. All you need to do is to port samba in application and enable some File System support in kernel if they are not enable.