There is nothing wrong with fasting on Mondays and Thursdays in order to make up fasts that you missed in Ramadaan, on condition that the fast be done with the intention of making up the missed days… Perhaps you will get two rewards together, the reward for making up the fast and the reward for observing a naafil fast, for the bounty of Allaah is immense. Even if you only get the reward for making up the missed fast, that is fine, because making up the missed fast is more important than observing a naafil fast. But if you intend to observe it as a voluntary fast and you do not intend to make up a missed fast, then you have not fulfilled the duty and you still have to fast the days that you missed in Ramadaan. And Allaah knows best. May Allaah bless our Prophet Muhammad and his family and companions.