If I build a PowerPoint VBA-Add-in under Office for Windows, will the created .ppam file work under Office for Mac?
I wouldn't count on it.
What will/won't work will depend to some extent on the Mac Office version.
In 2008, nothing will work. There's no VBA there.
in 2011, some stuff will break, and you'll have to write commandbar code for the UI, because it doesn't accept RibbonX.
In 2016, you get RibbonX more or less working, but the app is sandboxed ... you'll be severely restricted in where you can read/write files.
And of course, if you use any Win API calls, they won't work on Mac.