I'm at a loss at your use of terminology. Everything connected to your Libraries is in a runtime folder of one type or another that can be easily linked to any version of Poser. And that is the keyword, linked. You keep saying 'create a runtime' that implies to me that you are actually creating a new runtime folder. So that needs to be clarified further.

Where did you create your original runtime for Poser 10? Was it in a shared folder location or Program Files folder if you are using Windows? I can suggest a quick way to find a particular runtime folder, open P11 and navigate to your Library, selecting any particular runtime category. Hoover your cursor over a folder or file and you will get a popup that will show where that folder/file is located. It is the most convenient way to pinpoint content pathways.