Wow, I didn't know this kind of thing still existed...
We currently don't know what the support team might be able to do, the situation seems difficult for the company. If the previous owner does still have an account, he should be able to delete the Nabaztag from his account so you can register it, Well, it used to be that way, I don't know if it does still work that way...
BUT ! There are plenty ( well some ) alternative servers which owners can be actually contacted and may help you. For instance: http://openjabnab.fr/ojn_admiin or http://www.nabizdead.net/
You might want to give them a try if you want your Nabaztag up and running quick. For the past years, they've also proven to be more reliable than the official server...