Can I add a hydration/water filter/spray nozzle hose to a Watercell? What size fitting would I need?

The Watercell X uses a multi-function filler cap with an outer thread size of 63mm (the same as a Nalgene bottle cap / MSR water filter)
 
The internal thread of the multi-function cap (normally used to screw in the showerhead) is ¼” NPT. 

So - you can add an outlet hose to a Watercell X using a fitting: CPC makes a ¼” NPT to Quick Connect fitting that fits perfectly into the cap.

This fitting can then be connected to a tube with a hydration bite valve, or in on-line water filter, or even a spray nozzle.
 
 
Please note: If you are connecting a hose with a spray nozzle to a Watercell, and the Watercell will lie flat on a table or truck load bed, water pressure will only be generated by having the spray nozzle lower than the tabletop / vehicle; perhaps at ground level.