you can extend a drip system...however if the deck area is pots and hanging plants.....it will not work properly. The drip system for pots/hanging plants should run much shorter times than plants in the actual ground...therefore, if combined you will have either way too much water on your deck or if you adjust to lower times, not enough water to your regular plants.
best thing would be to put in a new system for just the deck. That will of course cause other issues such as:
Not enough pressure to push the water that high in the air?
Not enough emitters operating for the valve to shut off reliably...
good luck
Sprinkler Solutions, Inc.
Arizona and Colorado
www.sprinklersolutions.net