You have many unknowns here.
Was the system installed commercially?
Where on your property are you planning on adding zones in relation to where your existing zones are?
Do you have enough pressure with adaquate GPM to irrigate to an extended area?
How many of the smaller valve boxes did you find?
How many valves are there in each box?
Most common is to have a collection of valves in one box. If you have many (more than 5-6) zones you could have more underground valve boxes, each with a collection of valves. There is not a set standard here.
Less common is to have one valve in each underground box.
Start out by getting a couple of bids from commercial installers. Have the contractors explain how they would do it. At that point you will find out if what you are planning would work. BEFORE you start a large do-it-yourself project like this you would need to learn in detail what needs to be done.