yes... but it's an extensive job (and one i would only attempt with a bolt-on neck that i can easily detach from the rest of the instrument)... it also helps if you've got access to woodworking tools also (mainly a planer and electric sander)...
first you have to rip the frets out of the neck while causing the least amount of damage to the surrounding fretboard (easier said than done depending on how dry the wood is and how well fastened / glued the frets are)
once that's done you want to sand out any nicks and bumps from the edges of the fret grooves... then you want to fill the gaps with wood filler... re-sand the neck so that it's smooth and FLAT (even a smooth bump will tweak your sound in the bad way)
once you're satisfied with the smoothness of the neck you have to apply some sort of finish or varnish to the fretboard and voila... fretless bass
personally i've done this with an electric guitar... it's still in peices at a friends house in another province... i'm very lazy...
these links seem a little more professional than my ramblings so they might help you out a little more...
http://www.mikelull.com/fretless%20c...onversion1.htm
http://bassland.net/howto1.htm