I was hoping someone had a simpler solution, but this one seems to work.
1. Create a file that has all of the sources, but no people. I will refer to this as the master source list.
To do this, open the file that has all of the sources. Then use File menu > Save as ... to create a COPY of this file. Maybe call it SourceList.paf
Then File menu > Export > Select and select everyone. Now click the Delete button, to delete all of the people.
You will now have a file that contains only the list of sources.
2. Each time you want to have these sources available, create a COPY of the source list file and import a gedcom from the database where you want to use these sources. Now you will have the people and the sources you wish to use in the same file.
3. You can add additional sources to your master source list, by adding an individual to the empty file, creating sources, and then deleting the individual.
There are probably easier ways to do what you are asking, but this seemed to work for me. I wasn't sure the sources would still be there when I deleted the people, but they were there. And I was able to use them for a test individual that I added.