Is it permissible to fast the remaining make-up days after having observed supererogatory fasts in Rajab and Sha'ban?

You must make up for the days you missed fasting in previous Ramadans. There is no expiation for delaying the make-up fast until you became aware of the prohibition of delaying it. After you learned about the obligation of making up the fasts, you must make up all the missed days before the next Ramadan arrives. If Ramadan comes and you have not made up what you owe, then an expiation for delay is obligatory for each remaining day, unless the delay was due to a legitimate excuse such as illness. Scholars have differed on the permissibility of voluntary fasting for someone who has make-up fasts due, and the preponderant opinion is that it is permissible. However, this permissibility is restricted to the condition that the remaining period before Ramadan is longer than the days of make-up fasts; otherwise, it is obligatory to prioritize making up the fasts.