Is it possible to get into Psychology research after a PhD in HCI (human computer interaction)?
I've met people who were Computer Science PhDs and actually were part of Psychology departments. 

If you did HCI, I think it gets harder to do Psychology, after all is a whole study area by itself, since yours was more specific.

My advice would be to find a laboratory that has both of them, and that way you can get involved in some projects so you can learn other things about psychology

I do not presume to know about the filed, but I'm guessing there are multiple laboratories that might have a psychologist as well as a computer scientists. 

Essentially, build up your creed as a Psychologist by going somewhere where there is interaction between the two fields.