How to use Capital letter in user account?

Hi There.
When user account is created, user can not Capital letter in user name. I guess most of people will start their name with capital letter. But message will appear: "Only lowercase letters (a-z) and numbers are allowed." Now...

1 - Is there a solution to overirde this? I found someone ask about ti a year ago, but still now solution. (https://premium.wpmudev.org/forums/topic/way-to-allow-uppercase-usernames)

2 - If above is not possible, how can I easily add text above the user name filed "Please use only lower case letters and numbers." So user is not annoyed by system.

Thanks a lot.
Cheers,
Dean