Is it permissible to supplicate in prostration immediately before the Taslim in a Fard (obligatory) or Nafl (supererogatory) prayer, and what is its ruling in a Fard prayer?

It is not permissible to add prostrations in prayer for the sake of supplication, whether in obligatory or supererogatory prayers, because deliberately adding to the actions of prayer invalidates it. However, supplication during prostration is permissible and encouraged, and it is best to use the authentic prescribed supplications.