"Add to list" function

Is there a way to add songs to the "List" without using the "+" button?

When I drag a new file it clears the list.

Like drag&drop from the explorer while pressing the Ctrl-key?
You will notice that the mouse cursor then gets a little plus-sign.

Yeah, exactly that. Thanks for the tip. Is there a way to make that default behaviour?

As this is more or less like the Windows Explorer behaviour (which adds the selected files to the destination when you use the Ctrl key during drag&drop) I do not see it as a good move to change it.
Anyway: currently there is no option to modify this behaviour.

Yes, BUT only as default for ALL windows programs, not only for Mp3tag:

Sorry, just tested: That doesn't work for Mp3tag...
It works fine for copy/move files in windows explorer. But Mp3tag seems to check the drop target zone separately.

Update - How to do this on Mac version? Control does not work and just brings up menu.

I would say that you could try that yourself. I don't have a mac - what about the Apple key?

no such thing for that

can a mod please move to mac category?

Please have a look at the Mac documentation:

I mean there is no "apple key"

That may be as it is but there is the documentation that answers your question in full extent - which, if it had been read as requested, would have made the whole thread superfluous.