If you have access to a machinist (your uncle), I'd look into having him make a copy of your existing crank.

Welding extensions on cheap (???) ebay cranks would likely end up costing more than materials and labour for a good welder to add extentions, tap them for pedal threads, and heat-treat the crank so it's strong. I also don't know anything about how easy it is to weld a piece of alu to a forged crank. On top of that, since you're shelling out money for the welding, you'd want it ground down nicely and possibly finished... so yeah, I think a cheaper and better solution would be machined custom cranks by your uncle. You can have them made to whatever BCD pattern you'd like.