Three problems:
One - Euroleague is already underway, so scheduling would be a nightmare.
two - Champions League has relegation & promotion. There's no feasible way to work that into the NBA system, but it doesn't make sense to have a fixed group of X NBA teams and Y 'foreign' teams.
three - nine times out of ten most of the NBA teams would smoke even the best European league teams, particularly if the Euro teams have to fly to the US and play by NBA rules with NBA referees. It wouldn't be that fun to watch, compared to the Olympics or the off-season friendlies.