Well, if you manage to do that, good luck; but because hardly anyone would be able to exclude the chance of implementing any unfair advantages, I believe the staff may not appreciate such a modified client logging in to the official game server.
Not a problem for the development base data on a local game server, or if you are able to create a whole own game content, so your modification becomes a separate game: Then you should be welcome to combine the general gaming engine with a different graphics engine, I believe. The PlaneShift gaming engine is OpenSource; just the game content (what makes it "PlaneShift, the world of Yliakum") is copyrighted.