I made a plugin which you to allow to login Roundcube using your favorite username and password.
I also made a tool which convert a CSV file with account information to .config file at once to help your setting.
See my below Github page for details.
https://github.com/y-toy/virtuserpass
I made this tool for the company I work for. I want my company staff members to use their usual usernames and passwords instead of e-mail addresses and the passwords when they login Roundcube.