can a bellhousing be welded? (aluminum)

Yes it can but it can be a little difficult being that it's cast. (May want to crack in areas next to where you welded it. Make sure that whoever does it has it bolted in place, bevel at least half the material thickness and weld the outside first. That way if you have to have it unbolted to weld the inside, it's already in its natural place.
Pretty sure you have to put some heat into it before welding as well. A good machine/engine shop should be able to do it.