djoudi unfortunately, username rules cannot be easily changed at this time.
Allowing other characters leads to multiple issue, one being UTF-8 urls, another being the mentions system.
Related issues:
flarum/core557
flarum/core194
Maybe the Nickname changer extension can help you if you need a way to attach a display name to a user https://discuss.flarum.org/d/21238-nickname-changer
