Is it permissible to complete the 'Isha prayer with the intention of a supererogatory prayer behind an Imam who is praying Maghrib, or must one salam with him after the third rak'ah?

You should have completed the prayer with four rak'ahs and not have made the taslim with the Imam in the third rak'ah. This is because performing a voluntary prayer with three rak'ahs and repeating the Maghrib prayer is disliked (makruh). The Maghrib prayer is not sunnah to be repeated, because a repeated prayer is a supererogatory act, and supererogatory acts are not performed with an odd number (witr). Furthermore, performing a voluntary prayer with three rak'ahs during the day has no basis in Islamic law (Shari'ah).