I'm sure this is something simple, but, i can't figure it out

How do I make my builder summon buildings rather than actually building them?

You mean place them like UD does, or something like the Barracks item that the human campaign has or whatever?

I guess the simple idiot-proof way is to just base the custom unit off the original builder that has the construction type you want.

I think that all you have to do is check the Undead field on the worker unit at the Stats - Unit Classification field.

I thought is is hard-coded for acolyte :] I would base my builder off aco.