Are you looking for a rpmsign --resign equivalent for Debian packages?
Have you looked at debsign and wondered why you can’t run it on a .deb?
Did you try to run dpkg-sig only to have it complain about BADSIG after signing your packages?
Does it all make no sense?
That’s because you don’t sign Debian packages. You sign the repository index that contains checksums for the packages. Tools like reprepro, when appropriately configured, handle this for you.
So the reason you can’t sign or re-sign those existing Debian packages you built is that you’re not supposed to.
At least, that’s true in 2017. If you’re reading this from the future make sure you check in case things have changed.