Hi Chris and welcome to moodle.org
You can indeed choose particular entries from any number of secondary glossaries in the course and add them to the main glossary.
After creating a main glossary, if you then go to a secondary glossary, you'll notice next to each entry there is an 'Export to main glossary' icon (arrow with line under it).
For more information on what you can do with glossary entries, please see the documentation Using Glossary.