Attach foots to a rigged character
 

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.

Either use a constraint OR combine the meshes, don't do both. If you combine the meshes and merge the geometry together, might be best to redo the skinning of the skeleton and body. Simplest would be just to parent constraint the foot geo to a foot bone and don't group or merge the geometry together.