is it possible to fully convert a passive bass to an active? ie: giving it active pickups (which is obvious, i know this bit), active preamp, a battery box, more tone and volume nobs, etc?
You don't need active pickups. Most active basses have passive pickups and an active preamp/EQ. Usually an active upgrade comes with all the parts you need. On some basses you may need to modify the electronics cavity, but usually you can get sets that don't require mods (on Fenders). The J-Retro seems to be popular among Talkbassers, at least for Fender-style basses.
With a little skill and care, you can do it yourself. I just converted an inexpensive Samick fretless 5 into a tone monster with a Carvin preamp. Not to costly and excellent construction made it a pretty good deal. Works GREAT! IT's going to take some tools and the ability to solder but it can be done.
You might also consider an outboard preamp like the Sadowsky. No re-wiring to do if you go that route.