stewaran you may find it easiest to book +Qatar and then cancel STPC. They are two separate offers with very different terms and conditions.
With +Qatar you can choose your hotel from a list of available hotels but with STPC QR allocate the hotel and are not usually very flexible with regard to changing hotel on arrival.
Whilst I have not done so I think you can apply to reserve +Qatar without cancelling your STPC. When you have +Qatar confirmed then cancel your STPC.