dehetoxic you can indeed book a reward flight for Qmiles, later cancel it and get yor Qmiles refunded. But not Qcredits.
See http://www.qatarairways.com/Privileg...ge?mid=5&sid=1 for more details. Do not wait until the very last minute since the cancellation fee increases closer to departure and you could lose your Qmiles. You may also find it useful to look at the Award section of the PC rules.