Sorry for the delayed response; I recently started a new job and have been spread pretty thin, so I'm just clearing my inbox now. :-)
Thank you for using -- and improving -- my plugin! I'm always excited to see how other folks are using it and will definitely check out some of the changes you've made.
Best thing to do would be to fork the plugin on GitHub: https://github.com/sscovil/csv-to-sorttable ...then you could make changes, push them up to your forked repository and submit a pull request.
If none of that made any sense and you're not familiar with version control, feel free to just email me a copy of your modified plugin. I'll review it and see what changes should be merged into the main branch.
Also, I'm currently working on a new plugin that combines the functionality of this and another plugin I wrote a while back -- http://wordpress.org/plugins/sorttable-post/ -- and things like customer header rows are definitely something I was planning to add to that. If you're interested, maybe we can collaborate on that as well...