Thanks for quick reply.
I know I can define "UDC custom properties" and use them in security rules, but the advantage of "QMC custom properties" is that any RootAdmin user can modify them in QMC without having to touch the Excel file and reload. Moreover, one can see the values in the "Users" tab by adding corresponding columns in the UI.
So QMC custom properties are nice to use, except I cannot set/update them without being in the QMC and I cannot set values for a new user before he is listed in the QMC.
These 2 last points are made possible by UDC, that's why I would like to combine both and set "QMC custom properties" via UDC...