I want to extend an existing data format

TexturePacker comes with 2 types of exporters:

Internal Exporters

These exporters are compiled into TexturePacker and can't be modified.

Template based exporters

These exporters are based on a simple template language allowing you to change the output format

A good starting point for you is to use one of the existing exporters (they are inside TexturePacker's application folder in a subfolder called exporters. Pick the one that suits your needs and copy it into a new myexporters folder somewhere outside TexturePacker's path.

Point the exporter path in preferences to that location. Check out the TexturePacker for more information about how to modify the data format.