How do you add ingredients to fudge without stiring it?
Butter is typically added when the fudge is first taken off the heat- but it isn't mixed in. The butter is allowed to melt across the surface to keep it from forming a skin on top.

Vanilla, nuts, and all other additions are mixed in at the end of the cooling period when the fudge is stirred.

Alton Brown, with some help from Shirley Corriher, explained the process well.