I would try to stay away from that - even if it was a steel stem. Sure, a stem is thick walled enough to be welded w/o too much concern for burn-through. But stems are rarely nicely cylindrical, so getting the cuts to line up is likely to be quite time consuming.
And if it was me, I'd like a sleeve inside for reinforcement. also tricky with non cylindrical pieces.

Alloy - I wouldn't even try.

But mainly I don't think it'd need to come to that - unless you prove me wrong with measurements and pics.