Yes, Pan is in DBZ, but only in the final episodes during the tournament. She has few moves, and I don't think any were named, though I could check my manga again(I have all 42). She was also really young.
Pan is older in DBGT, but is very very young and tiny in DBZ when she appears.
I don't think it would be viable to put her in, but who knows?
I think(I don't KNOW for sure, I'm not the dev team) that the dev team would perfer to add characters that had major plot points in the series. It would also be alot less time consuming for all the modeling and coding that would have to be put in. I mean in DragonBall Legendary Warriors for Sega Saturn, there were ALL of the DBZ characters, from Pikkon, to Zarbon and Master Roshi. Now THAT's alot of coding, but then again, it was a japanese only release
And they had funding
Cheers.