Not in PyMol I guess.
Another idea would be to just homology model the residues (backbone) in. Maybe with a webserver like: SwissModel or CaspR if they can do just that part.
I know What If can do just the loops with the DGINSS option in the
Jurgen F. Doreleijers, Ph.D.
BMRB, Univ. of Wisconsin-Madison, WI, USA