Yes, it's definitely possible, just as you described, but how easily it could happen would depend on exactly how it works with the alien biology. Like if it's any fluid at all that can impregnate, or something very specific, and how far it needs to travel into the host's body. These are all factors that would determine exactly how involved their activity would have to be for it to happen.

Stella mentioned that it is unlikely for female half aliens to impregnate other sims, but it was possible. She didn't explain exactly what needs to take place for it to occur. (Of course, Jill doesn't know that either, so she wouldn't have any idea about how to prevent it.)