Well if there is no other source/binary available then you have to uncompress the deb/rpm file (both have some utility for that), look up the dependencies from somewhere,
copy the extracted files over to the nix output dir, and patch-elf all the binaries to link to the nix-version of the dynamic lib.
This sounds more difficult than it is, the result will be a copied over binary file that has its “libc.so” and other dynamic libs replaced in the ELF-header with “/nix/store/hdjdewuieu737-libc/libc.so”. I recommend looking up a package in nixpkgs which has a similar install story, that’s the easiest way to write a new package.
In case you only want to run it locally https://github.com/thiagokokada/nix-alien and similar programs work fine with the binary.