@Marcusb - This is @Gene 's area of expertise.
It looks like the solenoid for you system is 120 VAC powered. So you have an option: a) if you want a master valve in the system, replace the one existing solenoid with a 24 VAC solenoid or b) if you don’t want a master valve in the system, just open the existing valve manually using the top handle.
Replace the existing index valve with a 5 (I’ guessing here on the number of zones) port manifold system (Orbit makes a modular system).
Acquire the external Rachio enclosure and use the power for the existing Intermatic timer to power the encloser. I’m assuming this is 120 VAC and not 208 VAC as there is not a pump involved - but do check the label on the Intermatic to confirm the voltage it is getting.
What is different about this install is there is not a pump that the Intermatic timer controls, as it appears you are using a municipal water supply source that has constant pressure.