How to import multiple *.mbox files (folder tree) at once and not only a single at a time?

I could successfully import a (=ONE) single *.mbox file into eMClient.

But what if I have a (big) dir tree full of multiple *.mbox message files?

How can I import them in ONE step ?

With *.eml file dir tree this is possible. But for *.mbox dir tree I haven’t found a corresponding method.

How does this work?