[Nix-dev] Resolve conflicting packages by overloading?

[Joel Moberg]

If I install a package that provides a resource that already exist in
the environment I get a conflict, and from nix-env I get the
suggestion to change the priority.
Instead of setting priority, is there a way to instead specify which
resources in a package that are allowed to be overloaded in the user
environment?

If this feature does not exist, could it be implemented?