Additional validation for username field

Hi.

I am Antonio, developer of Joomdle. Joomdle integrates with Easysocial in several ways (sorry, still pending to list our plugins in your app directory).

One of the things Joomdle does is to make sure username entered on registration is valid for Moodle.
For this, we have a user plugin that works great with normal Joomla registration.
But this plugin won't work fine with Easysocial registration process. That's normal, as the process is different. For example, we needed to create a different plugin to have the same funcionality working with Jomsocial.

So, what I need is to be able to create a new plugin/app to add extra validation to the username field.
I started by looking into joomla_username field app, as I thought I should create new plugin based on this one.
But then I noticed that this new plugin would need to create a full new "username" field, so admin would need to modify login form to replace one for the other. And I think you cannot do that, as username field is mandatory and Easysocial won't let you remove it.

So, my question is: is there a way to add extra validation to an existing field?

Thanks in advance,
Antonio