Someone wonders:
He missed all of his fasts when he was younger, the wajib ones.
2 years I think he said.
He says he is unable to fast in Rajab because he has too many qadhaa days to make up, his question is, can he fast with with the intention of making up fasts and fast for Rajab at the same time?
He also notes that one of his teachers said that he has to do qadhaa first before he can do the ones in Rajab.

One cannot observe a Mustahab fast while having Qadha fasts to make up. What he can do is offer the Qadha in Rajab and ask Allah to give him the reward of fasting in Rajab.