From my understanding you cannot override core administrator pages. But you can add your own single_pages to any of the admin areas. Of course you can go in and change the core code directly, it's just outside standard practices. I believe the file is located at /concrete/single_pages/dashboard/users/search.php and the controllers for changing what gets saved would be in the /concrete/controllers/single_page/dashboard/users/ directory.

But take this approach cautiously, I have personally never tried something like that, just a thought at how you could do it, but certainly not the most thorough way.

The frontend part I got (something similar as the select from language or timezone).
But saving is a problem (and using the concrete's own functions).

I think you can do that simply by creating a new user attribute. IF it is required on registration, it will appear on the add new user screen and you can choose a value from it that will be attached to the user being created

I don't want to fill in all the client names (via concrete interface). The complete list with names and id's are in a second database synced with the clients software.

The problem with this user attribute is:
- You can't edit the values that way, only the display names.
- If something changes in the external db you also have to edit the display name in concrete.
- Have to exactly match names (strings with special chars), its better to match id's.

If i could fill in key -> value it wouldn't be such a problem.

oh sorry, I missed the "dynamic" aspect of your question.
Then maybe you should create a new attribute type which would get the data from the db as you want it.

You could probably do it quickly enough by modifying the core select attribute type

I planned to use this (manual input) as a backup. It works but is not ideal. (it breaks when the client name changes in the external db)

Better to modify the get_ and update_ functions from the controller.
But still not found the right combination of code to do it.