Yes, you can change it right now, but it requires a minor code change. The following question in our common support questions area can help you to do that now. Additionally, in the upcoming 4.0 release (in July), we'll have a selectable option for how the automatic username should be created that will make this more convenient.
How to change the automatic username from Facebook in JFBConnect
Hope this helps, but if you have any questions, just let us know!
Facebook Only Username
