How to create empty username of ppp-secret?

Hello.

I want to put temporary ip-addresses to users, who fogot enter usernames:
18:43:47 l2tp,ppp,error <10.10.198.3>: user authentication failed

(Example error with non-empty username: 18:41:55 l2tp,ppp,error <10.10.228.128>: user 105323 authentication failed )

How to create ppp-account with empty username (without using Radius) ?