Should I ask my online PhD interview panel whether my partner can sit in?
I would find this odd and unprofessional. I recommend against this.

Also, I want to be able to talk freely and openly with you in an interview, and have you respond in kind, and I would be worried about how having your girlfriend on the line will affect this dynamic. (For example: I might worry that you would be distracted by thinking about how she is perceiving the interview, or trying to impress her.)

I might also be concerned that needing moral support for an interview this badly does not say much about your ability to handle the stresses of graduate school.