" I want to replace farfieldvector3d with farfieldexact from the full metalens simulation. " this is ok.
however, "I want to use the farfieldexact with summing method" you cannot do it, as the summing method requires periodic strucuture, and both farfieldvector3d and farfieldexact cannot be used for periodic structure. Periodic structure is graing-structure with isolated diffraction orders and support plane wave in nature.
I hope this is clear.