I don’t think there’s any simple facility for specifying your own RotamerSet for use in packing.
If you’re working on the C++ code level, you can approximate pack_rotamers_setup() yourself by replicating the function calls in the implementation from src/core/pack/pack_rotamers.cc:189. Instead of using the RotamerSets setup there, you can supply your own, constructed by whatever method you wish.