Is it worth installing a woodstove to existing furnace duct work?

There is not one answer. It depends on the layout of the ductwork and the woodstove you plan to use. Also, keep in mind that a gravity vent has a very different clearance requirement than a fan forced vent so it is possible with the wrong setup to create a fire hazard.

I'm certainly not an expert at this, but I know there are "wood burning add on furnaces". My rookie brain thinks that is what these stoves are made for. The link below is one example of one I found. I believe that I found one on the englander site as well.

http://doitbest.com/Wood+and+coal+heaters-U+S+Stove+Company-model-1537G-doitbest-sku-449081.dib

To connect my RSF to ductwork using the top gravity vent openings requires a special butterfly damper that will only open under the force of a fan. The reasoning is that the fan will dilute the hot air to reduce the temperature. The fan also has a high temp cutoff to prevent air that is too hot from entering the duct.

Back when I bought and installed my first RSF, I got 3 cords of nice seasoned Ash from an acquaintance who had his installation condemned. He had a wood burning furnace that gravity fed into his existing oil furnace ductwork but the ductwork was not rated for class B vent. Don't assume that because it is a wood furnace that it will have the same code requirements as an existing oil furnace.

Also, codes may vary by region WRT to using existing ductwork. In my area it is strictly against code to connect my wood stove to my existing natural gas furnace duct. I had to have separate ducting.