Is it permissible to pray the supererogatory (nafila) prayer in the mihrab? Thank you.

It is disliked for the Imam to perform supererogatory prayers in the mihrab (prayer niche) for fear that others might mistakenly think he is performing an obligatory prayer and follow him. Some scholars have considered this an indication of the Imam's ignorance. As for one who is not the Imam of the mosque, there is no dislike in his praying in the mihrab.