You cannot use Nix package definitions with Guix. You can, however, easily extend or modify Guix packages. New package definitions can inherit from existing ones and override fields that you may want to change.

There is also an API for rewriting dependencies recursively:

https://www.gnu.org/software/guix/manual/en/html_node/Defini...

You can also use Nix as a package manager on top of a Guix system (and vice versa).

Most Nix packages, yes, but the NixOS packages specifically (the ones that build Linux), probably not. NixOS and Guix have slightly different philosophies, e.g. about nonfree software (Guix is generally opposed to including it, although I'm not an expert on the details, while NixOS disallows it by default but only by default), and this affects e.g. device driver packages.