Is there a surgical procedure that can fix badly pronated feet (flat feet)?

YES!!!: There are many different surgical procedures to "fix" flat feet. See a podiatrist for diagnosis, x-rays and a surgical consultation. These procedures range from relatively simple, to very complex.

Pronation: Yes - and as already stated they can be relatively simple to very complex.

Yes if: ...Symptomatic; custom inserts ; all conservative rx has failed. C a foot ; ankle fellowship trained orthopedic surgeon.

Yes: There are many causes that the end result is a flat foot (pes planus) but those that have associated symptoms unresponsive to conservative therapy ( stretching achilles, strengthening of inversion and orthotics/bracing) will come to have surgery. There are as many surgical solutions as there are causes.