Facebook login then modify username question

Yes, there's a few ways to do what you're looking for. The demo site (which is a bit more out of date than we'd like... we're planning a refresh and update later in Q1) is using JFBConnect's "Automatic" registration flow. In that mode, the user account is automatically created and profile information is imported. There's other modes (with lots of customization options) as well:
* Mini-Registration - In this mode, the user goes through a JFBConnect registration page. Here, they can set their username, password, Name and other profile information that would be imported into our supported 3rd party extension. There are options to hide any of those fields that can be imported or automatically generated as well.. so your registration page could just be "Username" where password, name and other fields are silently imported. It's up to you.
* 3rd Party Registration - In this mode, the user is redirected to a 3rd party extension (currently, we support JomSocial and EasySocial for this option). The user goes through the standard registration in these extensions, but profile data is imported from their social network to prefill and/or hide fields that we can get information from.

It's up to you. There's a lot of flexibility with JFBConnect to fully customize your registration flow so that it can be as quick or long as you want or need for your site.

I hope that answers your questions.

Thanks,
Alex

Yup, you can do that without any problem. We have many, many Kunena users of JFBConnect.. you shouldn't run into any issues. Of course, if you do, we'll be here to help or explain however we can.

Thanks,
Alex