I don't have the HT502, so I can't test this. If you have some spare time and a patient household (or an empty house ), call your cell phone from the line attached to Port 1. Once the call is connected, use the hookflash on your voipo line as if taking a CW call. At the dialtone, dial ***70 to ring the other line. You can try an attended transfer (answer the line on the 2nd port before hanging up the line on the 1st port) or a blind transfer (once it starts ringing, hang up the first line) and see what happens.

Mind you, this approach will not help you in the situation you describe (transfer an existing call to ring on Line 2, while answering an incoming call), as one of your ports would have to be handling 3 calls (the call already in progress, the new incoming call, and the call that's initiated to transfer one of the other two calls). I can't imagine a way to do that without having a PBX system (e.g. Asterisk) or key system in addition to the ATA.