Thanks everybody.

I know about Meshlab (used it to create create the reduced bunny). I tried Atangeo and I like it. Since I have rather simple objects, I can get away with the free version. The importObj script works great too!

That's my source object:



Then I use Atangeo to reduce/simplify it (this messes it up pretty bad, but I want it that way :) )



Then I import it in MoI where I can even use the move command to snap the edges onto each other and together with the planar command and bool union to produce a solid!