How to add the extra pictures from the iMac to the external drive?

I see that Carbon Copy Cloner has gotten to be $50. The trusted app PowerPhotos ($40,) in addition to adding lots of functionality to Photos, includes letting you copy pictures and albums from one Library to another while maintaining the organization. It's so useful, that I keep PowerPhotos open any time I'm using Photos.

Of course, the free method would be to use the Mac's built-in Time Machine which, like what Old Toad said about CCC, automatically makes incremental copies to backups of the Mac. The odd thing about TM is that a disk volume (but not the whole disk) needs to be dedicated to Time Machine. With an APFS formatted drive, you can have several volumes with variable size, expanding when needed.

Use Carbon Copy Cloner to clone your working Photos library to the backup library on the external drive. It only copies those items that have been added or

changed and deletes those that have been removed from the working library.

CCC can be set to backup whenever it detects a change in the source library or on a hourly, daily or whenever bases.

They have a 30 day trial but not sure if it is restricted in the number of items it will backup. Create a test library and import about 10 photos next to your working library. Create a new empty library with the same name on the external drive. Then test CCC by backing up originally and then make some changes, i.e. edits, add photo, to the source test library.