@SP.,  @Shek
I haven't purchased Magix Soundpools from Producer Planet.  So, a question for clarification:
If I
1) copy those (Magix) Soundpools into my Music Maker Soundpool folder, and then
2) run the "Rescan Soundpools" function in the Settings window,
then those new Soundpools should show up in my list of available Soundpools.
Is that correct?

And any non-Magix sound files can only be loaded via the internal file browser or via drag and drop from Windows Explorer? (your words)
 

@gjonny 1) and 2): yes, that's correct.

ProducerPlanet and the Music Maker in-app-store are connected. You can unlock the contents in both of them.

So it's even easier to download the Soundpools directly from the Music Maker in-app-store than from ProducerPlanet, because they are automatically installed into the correct folders.

And in the past the downloads from ProducerPlanet were missing some files that are needed for the Song Maker AI feature, if I remember it correctly. These files are only included in the downloads from the in-app-store.

And any non-Magix sound files can only be loaded via the internal file browser or via drag and drop from Windows Explorer? (your words)

Yes. Alternatively, you can use some kind of sampler plugin.

Technically, you can build your own Soundpools, if you organize the files the same way they are organized in Magix Soundpools.