They both use the same daemon (guix-builder is a fork of nix-builder), so I think they could share packages given the hashing is the same. This hypothetical has not been implemented yet, I'd love it.

Their is an importer[0], but this is basic and just grabs the package metadata, it's up to the packager to implement the package still. Hopefully this is further automated in the future.

[0]: https://www.gnu.org/software/guix/manual/html_node/Invoking-...