Hello, I'm starting with rigging and I've a rigged character without feet. I've modeled a foot and I want to attach them with this character and move them together.

I don't know how I should do it, I tried this:
1. Select foot and leg rigg, then click on "Parent constraint"; then I combine the body with the foot and when I move the rig...foot and leg deforms.

How I should do it?

Thanks and sorry for my bad english.