I have often thought about this, but I obviosuly don't have the coding skills. I imagine it would take quite a lot of rewriting but, maybe by starting with the very basic routines, fixing them, then expanding little by little, it's probably feasible, right?
Another option might be using the Tutorbot base, which is probably a little easier to play around with, since its more basic.
There was an Hexen2 bot, though :
http://cronos.planetquake.gamespy.com/ ... but I don't remember how well it played. Might be easier to work on it as a base? (implementing Frikbots/TutorBots features back into it?)