You can check your log for more information on why the import failed.
There's not a way to combine a skeleton as you described. A skeleton is made up of a lot of references to the various parts, it would get very complex to merge them. There are also invariants that must be upheld, like rules for name uniqueness.
Ideally if you share a project for multiple people to work on, they would change only animations, not rigging in setup mode. Then you can merge them using import project, animations. When rigging changes are needed, the master project would be updated and provided to the multiple people again.