Is it possible to mod it so HK can use a lightsaber.and maybe force powers,but mainly To use a lightsaber.

Droid with lightsaber... don't think so
you can't equip even with a sword

Yes, you certainly can, just so long as little things like "logic" and "animations" don't bother you. You can give the droids lightsaber feats, however since no animations were created for droids with lightsabers, they'll just stand there doing nothing. The game engine will still process damage accordingly, but "the show" will be non-existent.