From time to time they have a Champions tournament along these lines, I remember watching the Wildcats play Houston and Vasgo De Gama years ago in a tournament along these lines. Sixers played in one of these in the late 90's.
An ongoing competition like the Champions League would be great but it's never going to happen for the following reasons:
1. Distance - the Champs League works as it's restricted to European teams and therefore no match is more than a 3 hour flight away, I can't imagine the Sixers flying for 30 hours to play a mid week fixture against a French side.
2. Time to play - NBA teams play between 3 and 5 already each week, there's no way could fit any more into their schedule.
I'm sure there's more reasons but I think I'm just saying things people already know.
Regards,
Captain Obvious (heading home to put his feet up and watch Liverpool dominate Galatasary in the Champions league)