Good Morning:

I am about to install wiring from my house to my back deck.

I have an existing outlet which I would like to know if I can "hook into" it, and run the wire through a conduit to my deck as I wish to install a GFI outlet on the deck, or do I have to install a separate breaker switch?

No, extending the existing circuit will be fine. A separate circuit is not necessary just for a convenience outlet. If the new outlet is for some large appliance that will overload the existing circuit then a new circuit would be needed.