The "Add Existing Series" tool should only be used when a camera card was not used (or doesn't exist in the series to be moved) and the Subject or Group you're adding does exist in the database (if it doesn't, use the "Add NEW Subject Series" or "Add NEW Group Series" tool).
If you used a camera card but the card was not read properly — use the "Fix Mis-Scan" tool.
The "Add Existing Series" tool will create a new series that you can manually drop photos into and will contain all connected Subject or Group data.
Here's how you do it:
1. Click on the "Add Existing Series" button in the Inspector toolbar.
2. Locate Subject or Group record using search.
3. Gallery is now created and photos can be added.
4. That's it! You're done!