How to connect additional usb devices

Yes you can. Depending on what you want to connect an active (i.e. powered) USB hub would make sense.

I'd like to connect a second Touch Screen Monitor.  The 4 USB on our IonXe are currently:

1. Mouse
2. Keyboard
3. Touchscreen No.1
4. Eos 40x Fader Wing

Will a passive USB Hub work fine for that?

I'd like to connect a second Touch Screen Monitor.  The 4 USB on our IonXe are currently:

1. Mouse
2. Keyboard
3. Touchscreen No.1
4. Eos 40x Fader Wing

Will a passive USB Hub work fine for that?

You might also consider daisy-chaining the 2 touch screens. Keep #3 as above and then connect the second touch screen to a USB port on the first monitor. 

combine the mouse and keyboard on a passive hub then that leaves a port free for your 2nd monitor