Hi Ryan,
Are you planning to source notifications that are in a separate database from uPortal, or are you simply looking to store notifications from other sources?
You will want to define database details in a properties file, like those found in global.properties.
Then you will need to add overridesContext.xml from [1] under overlays/NotificationPortlet/src/main/resource/properties/ in your uPortal-start repo.
In that context file, you would add beans as needed.
You will need to create a new Java class that extends AnyRowResourceContentJdbcNotificationService to supply your query, etc. IIRC.
I may be able to write up a blog post on this process, but it would be a few weeks out. Let me know if you are interested.
Best,
--bjagg