Hmmm.

I just booked BUF-ATL-AMS-(stopover)-LCY, and CDG-DTW-BUF return. I asked about doing something like NCE-CDG-(stopover)-DTW-BUF and was told it's OK, since both AMS and CDG are gateway cities.

Not too sure about that NCE-CDG leg, though, since it's quite a distance.