Is it possible to search for a Tag, and then Tag some of these photos with an additional tag?

Is this something could be done via certain themes that allow tagging on the front end?

It does not look like it would be possible on the back end.

This feature is part of the ZenPhoto20 branch of the software.

I was looking for advice on a solution within ZP. Not interested in switching to new CMS.

No problem, stick with the features that you have. But ZP20 is not an offshoot of Zenphoto, Much the same as when the community took over Zenphoto from its origins.

@tplowe56: Sorry this is currently not possible with Zenphoto.
Until now not a feature we had thought about.
We try to consider it for the future.