Yes, you can use a momentary Foot Switch (FC4 or FC5 type) plugged into the Assignable Foot Switch jack on the S90 XS back panel. This can be set to toggle the Arp Switch On/Off.
Set the FS = Arp Sw
(This is located just above cc95 on the list of options)
Now, we would be remiss if we did not state the following: this will activate the Master Arp Switch, a Part or Parts must have their Part Arp Switch set to On, and you must trigger a note or notes on the keyboard in order for an arpeggio to start. That is always true. Assigning FS = Arp Sw will just activate the main Arp Switch.