Is it permissible to count the fasting days that were intended for the six days of Shawwal as make-up (qadaa') if the fasting of the six days was not completed?

It is not permissible for someone fasting voluntarily to change their intention after completing their fast to an intention of making up a missed fast (qada'), nor to change it during the day to a qada' fast after having started as a voluntary one. Changing the intention after finishing and completing an act of worship is not valid. The days fasted in Shawwal are not counted as making up for Ramadan, rather they are based on the intention of fasting six days of Shawwal. Not completing them does not invalidate the reward for what was already fasted, and it is recommended to complete them, even if separately.