The current method of appending tags through creating an action is extremely cumbersome, especially when I want to append different text to different tags of different groups of files.
It'd be nice if I could append text to tags directly in the sidebar as I show in the screenshot.
You can also use Convert Tag -> Tag (Alt + 5)
The format string would be the same as in your example.
To add text to the tag TITLE, it would look like this:
In the Preview (green rectangle) you can see immediately the result.
After pressing OK this result will be written into the tag in Field (TITLE in this example).
Aha!
That might just do the trick!
Amazing how long I've been using MP3Tag and how little I know of its features!