I managed working a full time job on a flexible roster and worked part time as an FI on days off and after work (if I was on a short shift) so it can be done. I'd get my roster mid month for the following month, I'd write in my availability and if there was work I'd get it. In summer I'd get a good amount of hours in.
So it is possible and I'd imagine if your full time job is fixed days and times then it would be even easier.