I think it should work if you treat things as a single continuous chain. Just ignore the chainbreak, and concatenate the sequences together. (In the proper order.)
I think that should work without any major alteration of the template PDB, but if worse comes to worse, you can always rename/renumber everything in the template PDB to be the same chain. The partial_threading application won’t care about bad geometry at the join (particularly if it’s spit up/not aligned to the target sequence), and it’s rather agnostic to the naming of the input residues, provided they’re in the correct order.