You can add individual files or folders to the current list by drag&drop plus pressing the Ctrl-key from an application that supports this interaction.
This has been requested a number of times (just use "instances" as search word), e.g. here:
so what makes your workflow special so that you need further MP3tag instances - and how is this workflow not covered by sorting and filtering?
yes, you can go into a folder on your file browser on windows and select multiple files using CTRL key and right-click and choose the mp3tag option, however once it loads those if you wanted to add certain specific files from another folder, there isnt an option for it only to add directory and you cant drag&drop them in because it overrides the current list with the files you drop in.
its just because sometimes ill be editing a list of files and will remember something i really need to do in another file that's similar to something I'm currently working on in another place and don't want to interrupt my current work flow but want to quickly edit another file. Also it kind of ties into there only being an option to add directories and not individual files. But yeah, sometimes I just want to have multiple windows so i don't have to change my current project in any way.
The procedure is as follows:
select the files in the source application (this could be iTunes, the WIndows Explorer, Foobar2000 ... probably more - but not Windows Media Player).
Drag them to the MP3tag window and hover there, do not release the mouse button.
Now press the ctrl-key
The mouse cursor will change and get a little added plus.
Now drop the files and these files will be added to the already loaded files.