Some years ago I was running an AD&D 1E campaign. I had in a lich controlled dungeon a woman who wasn’t exactly in that universe.
I decided to take the Traders, mentioned in GURPS Classic Aliens book on page 100 and modify them so they look human, and do some of the same things. They are not related to humanity, nor humaniti, in any way. They just decided, for reasons of their own, to look like them.
They have a home area, but not in this dimension.
Not all parts of them are visible at the same time, so parts of their arms, legs, faces, might not be visible. Or slowly fade away, or fade back in to visibility. This could be a bit disconcerting to whomever they are talking to.
They don’t know everything, although they may pretend they do. Or they will be helpful like the one in Sraa Keep.
As for the tech level mentioned on my other site, that may be too low or too high.
Some items mentioned in the GURPS book wont work for these characters, and some of it will. If I feel the need to go into more detail, I’ll add it later on.