Dreaming in Technicolour? Nope, that sounds like a good idea.
Unfortunately PetroCan (and Speakout – same company) does not sell the SIM cards separately from the phone.
Fortunately there are people on this board selling SIM cards, and once in a while you may see some selling a PetroCan SIM as well.
In case you can't find someone selling a PetroCan SIM, you can always buy a phone+SIM and sell the phone for a couple of bucks.
As far as porting your number, with a new PetroCan card you can do it, and I assume that you can do that with a existing PetroCan account as well.
And of course you can use the SIM card with any unlocked GSM phone.