If you´re crafty you can make your own jacket extension or babywearing coat.
Here are a few DIY guides and directions:
http://dineandtwine.wordpress.com/2014/09/23/baby-belly-maternitybabywearing-jacket-extension-panel/
And a collection of tutorials for babywearing coats / coat extender solutions:
https://swoodsonsays.com/diy-babywearing-coats/
This is a classic tutorial from http://www.kiddiesgames.com/jacketinserts/
If you want to use your own existing coats and jackets, then the main task and problem is to find the matching zippers that can be connected to your coat´s zipper. We are happy to support your DIY project by providing the right zippers. You might even use our Kumja zip adapter system for your own jacket extender to be able to use it for any jacket. Contact us if you have questions :) Or have a look at Kumja.de/en