Have you seen the Araka Kalai publication? This has a unique Ivashu that is a shape changer. It's not a 'species', but might be useful. Other shape changing things might be spirits and people with 'skin changer' talents?

All editions of the HM system have a version of what's called the Transmutation "psionic" talent, an innate ability to shape change. In HMK, KP's most recent version, such innate abilities are renamed "arcane" talents, but the concept is similar to if not exactly the same as in the other editions: the ability to change into an animal. This isn't a 'disease' or an 'affliction', per se (although certain cultures might view it that way), but an ability the character controls. HMK Arcane Talents are discussed starting on page 259, with Transmutation detailed on page 264.

However, what it sounds like to me—if I'm recalling my Star Trek lore correctly!—might better be modeled by creating a new arcane talent based upon the Neutral spell called Mask in HMK (page 253). Here, the ability would duplicate the appearance of someone—change their facial characteristics etc—rather than actually transform the person into a different animal.

Finally, you might combine the above references—including Balesir's suggestion—into something closer to your liking. In the end, broadly speaking, it seems the easiest solution would be to make it an arcane talent that your PCs would possess.

I'd be interested to see what you come up with—come back here and share it with us!

You're welcome, Armandrik—I'm looking forward to seeing what you devise!