Is it permissible to perform a supererogatory prayer, such as the supererogatory prayer accompanying Maghrib, before the Asr prayer?

If you are obliged to pray Asr, then it must be made up before Maghrib according to the majority of scholars. If you delayed it out of ignorance, then there is no re-performing of the prayer nor is there sin upon you. As for the Sunnah (definition) prayer of Maghrib, it is not permissible to perform it before making up Asr according to some scholars, while others permit it. Absolute supererogatory (nafl) prayers are not permissible before making up the Asr prayer.