Christian Schafers from GP3Station.de has e-mailed me to say that Infogrames Germany have said to him that it is "not clear" whether the GP3 2000 add-on will be released at all.
This follows previews in magazines and information about what will be in it. However, it is interesting that I have not seen ANYWHERE where you can pre-order it.
But, I am pretty sure that it will be released, even following these comments from Infogrames Germany.
If you can read German, GP3Station.de have a whole news article about it. (Unfortunately there is no English translation).