You can turn any regular document into a Library in a couple of different ways, depending on whether you’ve saved the original document locally or to a Workspace.
You can open any local or Workspace document and choose File > Add as Library… to turn it into a Library and add it to your Libraries.
In your browser, you can hover over the document’s thumbnail in your Workspace, click on the cog icon that appears, then choose Settings. In the Settings panel that opens, enable the Share Symbols, Styles and Color Variables option in the Use as Library section to turn your Workspace document into a Library. You can now add this via the Mac app’s Preferences.