sroger,

Two things.

1. If you add to a zone, you have to make sure you are not over-demanding water from that zone. All the heads will suffer. You need to know your pressure and water availability to that zone. Add up all your flow from your heads and future heads and do not exceed 80% of what you can deliver to that zone.

2. When you remove a head that increases the area you have to cover and reduces the amount of water to the new heads.

Run a mainline valve and create a knew zone that cover the additional area or adds to the existing areas if the original zones didn't cover well enough. Gosh JMHO. Aloha

if you are going to the trouble of tapping into an existing zone, why not just add head(s) to cover the dry area instead of messing with a hose? Follow Lehua13's advice about checking capacity, though.