Welcome back Thomas
... and a HAPPY NEW YEAR!!
I have investigated this in the past already and it took me a while to grab that out again
The short (sure a little unsatisfactory) answer: sorry, currently I see no way to add these fields into system emails sent from Joomla, except to touch the core code of it. This is imho an absolute "nogo" since this would get lost every time you update Joomla.
The longer (and hopefully more liked) answer: my tests with Joomla 3.x did not bring any light into this, but working since quite a while already on jOpenSim for Joomla4 and testing there (the reason for my delayed response) could finally give first very promising results.
In other words, jOpenSim 0.4.x will most probably get another new plugin that can handle what u asked for ... I just can not promise if this is already part of the first release of it, since it is already short before a possible ReleaseCandidate
Greetz
FoTo50