In general, any undead creature should join. By the way, when you start working with the campaigns of the Hammers of Fate, keep in mind that in the final mission of Suspicion (the second mission for Freya), the final cutscene is not shown. And this is, by the way, the only cutscene in which Kha-Belekh appears. Because of this, even in the Ashan Wiki, on the page with the biography of the Overlord, it is not mentioned that he appears in the Hammers of Fate. In general, there are quite a few cutscenes in the first part that are not shown, but there is an option to view the cutscenes in the settings, but there is no such option in the Hammers of Fate.

Added after 19 minutes
And by the way, you can make it so that mummies are raised from the mages; the biography of the mummies states that they are resurrected mages.