Its still very likely that it can be done some way - its just a matter of the expertise needed and the time it takes to work out and implement.
If it isnt possible however much time is spent on trying to get it to work, then the new site will have to be seperate.
The implications of this must not be over looked - the site would have far more de facto and delagated power if it has a seperate database, the idea of curial power there would be unlikely (a good thing), and many of the problems of Ogres.net would be repeated.
Seperate sites is better than nothing but sites linked by a joint user database would be far better.