Since you're using anti-siphon valves, I assume you don't have a backflow prevention device between your valve manifold and your main supply line. The anti-siphon feature of your valves is intended to prevent possibly contaminated water from being sucked back into the main line. This method of backflow control is now generally viewed with disfavor, and may not comply with applicable codes. If the valve is not positioned at the mandated height above the sprinkler or drip line, the anti-siphon feature is defeated. On the bright side, a simple $10 backflow preventer is usually all that is required for a soaker hose or drip line, and when you open the valve, the water will flow. You will also need an inexpensive pressure regulator. Putting this together may be a bit tricky, and you will need to bear in mind the incompatibility of hose threads and pipe threads as you select the requisite components.

lafutura, you have piqued my curiosity as to why you would want to install a perimeter drip line on a lawn that is already watered by a system. I would think that installing a gate valve or similar ahead of the drip zone would allow you to regulate the water flow into the new drip line. Concerning the other question, there ought to be a manifold on the premises. Either all the piping to the zones start out at a central location and are routed their separate ways or else all the valves (and piping) are in a central location. Another possibility is that a main 'hot line' was run from the water source and the zone lines (and valves) were tapped from it. A radical approach would be to disconnect the wire from the unidentified terminals (one at a time) at the clock and observe which area wilts. This is recommended only if you can critically observe your whole landscape daily. You want to spot the earliest symptom of wilt to avoid serious damage to you landscape.

One suggestion is to find a sprinkler head you don't need (like a popup sprinkler used for shrubs that are mature enough that they no longer need irragation) or you can install a new 1/2" outlet to your existing system. Then all you need to do is attach a drip head conversion kit. It allows you to run up to eight 1/4" microtubes from the single head and you can regulate the flow/pressure to each tube. You can buy the converter at the big box stores. I've used them for years and have had no problems with pressure. I've included a link to the conversion kit sold on the Drip Store. Here is a link that might be useful: Drip Store Head Conversion Kit

the main trouble with mixing drip and spray is water volume and run times. spray heads can put out 2.5 to 7.5 gallons per min. while drip emitters put out 2.5 to 5.0 gallons per hour. what this means for you is that you need to add enough emitters to give your garden enough water in the shorter run time your grass requires.