Dynamics 365 Online is a great platform for extending almost anything including system entities like systemuser. While it’s easy to customize this entity to add some simple additional information about the user, e.g. charge-out rate, induction training completion date, latest lasertag achievement, etc, it may be not a good idea, after all.
The problem is that systemuser entries and, specifically, their status, are controlled outside of Dynamics 365, by Azure Active Directory. During the routine updates it may cause some drama if, for example, a Space Invaders score needs to be corrected for someone who no longer has Dynamics 365 license and is therefore disabled. Data migration between the systems would be even more challenging with the only solution being to temporarily grant licenses, wait a bit for them to take effect, update what’s needed and then remove the licenses. Possible to automate but rather tedious and may require some extra licenses.
Instead, keep additional information about the users in a separate entity with 1:1 relationship to systemuser. Updates to inactive records? Easy done. Migration of the data for 700 disabled users? No problem. And if you ever wanted to keep some of the private information like their hobby or last year bonus, you can apply some Dynamics 365 security juice.