Extending an existing format
TexturePacker comes with 2 types of 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 Online Documentation for more information about how to modify the data format.