I’m pretty sure this function in <nixpkgs/lib/modules.nix> does exactly what you want:
538 # Convenient property used to transfer all definitions and their
539 # properties from one option to another. This property is useful for
540 # renaming options, and also for including properties from another module
541 # system, including sub-modules.
542 #
543 # { config, options, ... }:
544 #
545 # {
546 # # 'bar' might not always be defined in the current module-set.
547 # config.foo.enable = mkAliasDefinitions (options.bar.enable or {});
548 #
549 # # 'barbaz' has to be defined in the current module-set.
550 # config.foobar.paths = mkAliasDefinitions options.barbaz.paths;
551 # }
552 #
553 # Note, this is different than taking the value of the option and using it
554 # as a definition, as the new definition will not keep the mkOverride /
555 # mkDefault properties of the previous option.
556 #
557 mkAliasDefinitions = mkAliasAndWrapDefinitions id;
558 mkAliasAndWrapDefinitions = wrap: option:
559 mkIf (isOption option && option.isDefined) (wrap (mkMerge option.definitions));