adding a third CDJ to DJM250Mk2

Hello and already an excuse for my noob question :)

My current setup is a MacBook Pro 15" with Rekordbox, DJM250Mk2, 2xCDJ-400 and a DDJ-XP1. And some SDJ's for sound and monitoring. 

I am just a bedroom dj or on my office on fridays, so no real financial gain or other wishes.

On the DJM-250Mk2 there is a third port, being the AUX. and it had a Analogue RCA and digital USB switch.

Although there is no USB further connection, my setup is fully USB connected.

So de DDJ-XP1, DJM-250Mk2, 2xCDJ-400's are connected with only USB cables with an powered, USB 3.0 Hub which connects to my MacBook Pro.

Within Rekordbox i have the CDJ's on channel 1 and 2, but could i buy a Rekordbox compatible third CDJ and add it to an existing USB port on the 3.0 Hub and within Rekordbox select this third CDJ to channel 3 and put the selector switch on the DJM-250Mk2 on digital input?

So i would have beat sync or visual sync on Rekordbox screen and volume up and down through the volume button? So i could use a beatloop on this with only volume up and down, no EQ etc?

Ik hope someone can answer this. Because i would like to buy a new CDJ and use one existing CDJ, as an third option.

Thnaks in advance

Alexander