Is it permissible for the Imam to combine two prayers with the intention of a supererogatory (nafila) prayer, not a fard (obligatory) prayer, and would the prayer then be valid?

The worshiper must intend the obligatory prayer when commencing a fard (obligatory) prayer. Intending a supererogatory (nafl) prayer is not sufficient. Thus, if the Imam intends a supererogatory prayer, the obligatory prayers he led become invalid. As for 'azimah (resolve/determination), its meaning in Islamic law is something that is legislated from the outset, without being connected to a transient cause, and it possesses the utmost strength and certainty.