No, it's not possible to do this. At a technical level, it's not a big problem to write the code yourself, because the synthesized code is always very straightforward and magic-free. It's no worse than tedious to write your own replacement.
OTOH, it would certainly be good if the synthesized code could be used as a starting point for a given type's custom code, or even the synthesized code could be inspected. It's not easy right now because it's synthesized directly to a compiled form — the compiler doesn't actually produce the source code (AFAIK, which isn't very far, I admit).
You should re-ask this question over on forums.swift.org. It's a known issue there, but if you add your voice to the debate, that increases the chance that the Swift team will feel motivated to implement some solution. Even better, you are free (as is anyone) to get involved and work towards a solution that eventually gets incorporated into the public Swift release.