Is it permissible for someone who intended to pray Witr with one rak'ah and forgot the qunut supplication to add two rak'ahs in order to recite it?

The supplication of Qunoot in Witr prayer is a matter of differing opinions regarding its desirability. The preponderant opinion is that it is legislated in Witr prayer, both during Ramadan and outside of it, but it is not obligatory. Therefore, the prayer of one who omits it, whether intentionally or out of forgetfulness, is valid. It is not permissible to change the intention for one who intended Witr as a single rak'ah to make it three, according to many scholars. However, the Hanbali school of thought permits changing the intention in Witr, as it is permissible to add a third rak'ah for one who intended two rak'ahs then stood for a third out of forgetfulness. The more cautious approach is not to divert from the intention you have made.