Is it permissible to join a congregational prayer with the intention of making up unspecified missed prayers (such as the Asr prayer after performing the current Asr prayer)? And should the supererogatory (nafila) prayer be offered before or after this prayer?

There is no disagreement among scholars regarding the obligation to make up missed prayers due to a legitimate excuse, based on the Prophet's (peace be upon him) saying: "Whoever forgets a prayer, let him pray it when he remembers it." A prayer missed in this manner must be made up immediately and remains a debt upon the Muslim until it is performed. He incurs sin if he delays it without an excuse. However, it is permissible for one who owes a make-up prayer to delay it for an excuse, such as harm to his body or livelihood. It is permissible to make up a missed prayer behind a congregation performing a current prayer, and this is the issued fatwa, just as it is permissible to make up a prayer behind someone who is making up a different prayer. It is not permissible to engage in supererogatory prayers for one who has missed obligatory prayers, because making up obligatory prayers takes precedence over them. Some scholars, however, have made an exception for the regular sunnah prayers, especially the sunnah of Fajr and Witr.