I am running ReadyAPI 2.1.0 - not the latest, which is 2.2, but since no one responded yet, allow me to try.
I don't know of how to link back from SoapUI Pro to the Project.
I regard the project as a place to set up my APIs (I think you refer to these by "elements."), resources, methods and requests as templates, and the SoapUI for variations on these. In SoapUI Pro I clone the test cases and then edit the cases, maybe add groovy scripts as steps. The groovy scripts of course do not show up in the Project window since it seems to be for setting up REST step templates.
But if I understand correctly, you have a whole bunch of APIs - and duplicated endpoints when you can combine them and add numerous resources per API. I am working with my IDE and not a project file but I haven't been able to find how to move a resource between APIs, unfortunately. That is something I guess you want to do.
I can only think of biting the bullet and entering the resources by hand to the common element / API.
I think it's possible to edit the xml file for the project itself and shift all the common resources to the endpoint/API. But I don't like to monkey with the xml file for the project.