How to remap individual key on Apple Magic

How can I remap individual keys on the Magic Apple Keyboard for the iMac? I would guess this would be the same for Mac laptops, too.

My specific question: I have the extended Magic keyboard with number pad, German version which gives me all Umlaute, like äöüß. That's good. However, I use many UK/US spreadsheets where comma , is a list separator, and the dot . is a decimal point. The German keyboard uses the comma , as decimal point, and there isn't a dot . on the number pad. Soo ..., I want my text keyboard in German (with all Umlaute), but I want my number pad using English formatting (with a dot instead of comma).

So I want to remap the comma key , on the number pad to produce a dot . instead. The main keyboard must remain unchanged.

Can this be done? Standard system preferences don't seem to help. Older discussions here on forum don't seem to help 😟