Is is possible to retrofit a wood-burning stove into an existing gas central heating system?
Up the creek. What you are looking for is not that uncommon here in Maine. Wood fired furnaces and aux wood fired systems are getting more popular. Many are run together with another system such as you are looking to do. I am no expert on these things, but have two good friends that have put in external wood furnaces and had them plumbed into the domestic hot water and forced hot water heating system.  The key to the system is using a wood stove/furnace with a properly designed heat exchanger that easily works with your existing system. The theory is simple, wood preheats water going into your boiler. The hotter the preheated water is, the less your gas furnace has to work.

I saw several different systems at a home show last spring. If you don't have any HVAC experience, I'd really suggest checking out ready made systems at your local dealers. Creating something like this from scratch looks pretty difficult to me. A good dealer can show you what you will need and what options would work with your situation. Safety is always your first priority, so get some good advice, good equipment and you should be on the way to saving a lot of money in the long run.  Good luck...