Yes, NW could pick up one used ARJ from a European carrier (several of them are selling them off), but it's unlikely to happen. A scope caluse prevents NW from operating more than 36 69-seat aircraft, and I doubt NW would try to pick up just one used aircraft. The BAE 146s are older versions of the ARJ. The ARJs NW operates were meant to accomodate 85 passengers, but NW can't operate them with more than 69 otherwise they'd be mainline jets... that's why we find them so comfortable.