The openings might very well be threaded (after all, the iron pipe screws into the intlet, right?), then all you need is the appropriate size PVC male fitting (Im assuming the inlets are female) and some teflon tape.

The iron will probably come out pretty easily too, you could try a band wrench or if you get desparate drill holes thru the pipe and shove a screwdriver thru to give you some torque

I actually had something similar to that setup when we moved into our house........flex PVC pipe to brass gate valve to brass adapter into the pump. Luckily I needed to replace pump and filter so it all got tossed.