I tried this only with the Final Save which is generated automaticly after killing Sarevok. Two problems happened:
1. The character will loose the biography and will have multiplayer biography (starting with: "You have been the friend of CHARNAME...")
2. If the difficulty settings are at lowest (or lower than normal I guess) the HP will recalculated but I don't know how. For example I had a level 9 mage with 18 con (so 54 HP) after imported I've got 37. This won't happen though in normal mode.