Can it be done Technically, yes. I do not think you can currently do it (without rigging up stuff). (Ahhh, if I understand the question correctly)
It would be nice if you could cross various systems and wireless speakers/subs. Think this might make sense especially when combined with a transformer … I.e. axiom should consider it.
P.S. I have an N3 with a sub out. I bought it as a refurb (I like to think I got Mojo’s return). It was a bit too boomy indoors for my tastes (Mojo tells me they fixed it since …) outdoor on the deck it’s much better and now it’s used there or on the patio. Originally I was hoping that it would cross to the sub when a sub was plugged in but the jack was just an output … I.e. didn’t help the boom just augmented it