What is the ruling on combining prayers (Dhuhr and Asr, and Maghrib and Isha) for a non-traveler, and is it permissible for a Muslim to pray with the congregation with the intention of a supererogatory prayer and then repeat it at its original time?

If combining prayers is not for the stated excuses, then it is impermissible. It is better for you to leave after performing the present prayer so that you are not an aid to their transgression, and so that the ignorant are not deceived by your action. There is no harm in praying a supererogatory prayer behind someone who is performing a fard (obligatory) prayer.