Yes, it can be done, but it's a little difficult. Here's a guide that tells how to add a subtitle to an existing DVD, but I think you lose any menus you have. Perhaps it's possible to switch one subtitle for another and keep the menus.
http://forum.videohelp.com/topic245188.html
Also it might be easier for you to rip the DVD to your computer using dvddecrypter or DVDFab or something, and then convert it to Xvid using AutoGk. Then you could watch it with your .srt file.