You can’t extend the 0% interest period on a purchase card, once this ends, you will have to pay interest on your remaining balance. However, there is a clever way to avoid paying interest:
Enjoy using your 0% purchase card but make sure you are making the monthly minimum repayments. Before the 0% period ends, apply for a 0% balance transfer card. Once approved, shift your debt from the 0% purchase card onto the 0% balance transfer card. Pay off your debt on the balance transfer card at 0% interest. Make sure you pay it all off before this period ends.