It is actually rather hard to get sitebanned from Wikipedia and it's pretty easy to return.
Simply wait on the sidelines quietly without trying to evade the ban on sockpuppets and either let the ban expire or make a polite request to have it lifted. Promise to abide by policies and site standards.
If the editor has done those things, and hasn't done anything extraordinary (such as threatening bodily harm to another editor), then I'll support their return. If I weren't the blocking admin I'd open a dialog to make sure those standards have been satisfied.
I give barnstars to people who make productive contributions after making a legitimate return from a siteban.
Also, there is *no such thing* as a ban from GDFL licensure. People who get banned from Wikipedia and think they can do things better are welcome to start a new wiki and test their principles at it. They don't have to start from square one: Wikimedia's wiki software is free and Wikipedia's article content can be taken and adapted free of charge.
-Durova