One relatively low-cost solution is to get a WiFi smart power plug, put a 24VAC transformer onto it, and set up a zone as ‘HTTP zone’ on your OpenSprinkler to communicate with that WiFi power plug, which in turns switches on or off your valve. This solution (along with some other alternative solutions) are documented in this blog post:
https://opensprinkler.com/switch-pumps-with-opensprinkler/
Another option is to add a zone expander, but given the distance between your main controller and that valve, you would need to make a custom expander cable (it needs a minimum of 6 wires plus a common wire). This gives you 16 addition zones on your far side but making the custom cable can be cumbersome.