Username as number only

Good afternoon from Czech Republic,

I would like to ask you if exist any plugin (or how to do), which will automatically create username in wordpress registration as a numbers only.

For example:
– when new subscriber came to register to my web, his username will be randomly generated as a 12 digit number and there will be no possibility to change it.

Is that possible?

Thank you for your replies…

With kind regards… Stefan alias “Scharfheimlich”