There is acutuly a couple ways to handle this. The easiest is use a product called add a zone or quick link. Cost is around 80 to 140 for these devices. It's just a time saver realy. Better than digging and runninng irrigation wire from timer to valve. If it's easy access, just run a new wire.
First, your timer must be able to be jumped. Low end homeowner timers may not work for you. Most of these devices use the same common and positive leave. The device tricks the the valve using a simple relay system. Problem is: You have to use the same type watering, it can't be one valve a drip system( gph) one grass(gpm)
J-LC Landscaping & Maintenance Gardening
C-27 CA. Contr# 770044