Username only lowercase and numbers in WP multi

Hi Everybody,

We are using wordpress in multi sub-domains at work.
So we have administrator for each domain who can create users for subscribing editing contributing.
The problem is, our company edit the rule that all logins/usernames have to be like this “first-name.family-name” ex “barack.obama”… ??
But the administrator can not do it, only super admin can do it !
In sub-domain administrator mode, there is a message that says : “Only the lowercase letters a-z and numbers allowed”.

So we want to make it possible to admin to create users with first-name.family-name, but we try to search how to change it in the code, without finding it.

If someone can help, thanks !