Hello! Yes, you can gobble up the undead. Just as a heads up, some of the higher-ranking undead are deeply connected to the necromancer lieges who summoned them alive. Consuming one of those undead might briefly transfer the compulsion to obey their necromancer unto your character, until the undead meal has been fully digested. This only relates to a minority of undead dinners, however. Most undead sandwiches will not affect your character in this way.

However, some undead creatures will have died some time ago. Being summoned back alive does not typically replenish any of the flesh that has rotted or otherwise perished in the killing process — in other words, some of the undead might make for light snacks, rather than main courses.