First off, I modified V2.1 of the plugin and Kreg released V2.2 a few days later. I'd imagine the relevant parts of the code haven't changed too much, but if you're using the latest version you'll have to work through it and try and match up where the changes should be applied.
Also worth noting that any subsequent update of the plugin would probably need the changes reapplied, as they'd get overwritten by the 'standard' code.
Anyway here's what I did:
1. Using phpMyAdmin, inserted 3 new fields (named as follows) to the 'wp_dk_speakup_signatures' table:
All configured as follows:
(I positioned them just after 'custom_message' field).
2. Modified emailpetition.php and public.js which manages petition when used in page or post content.
3. Modified widget.php and widget.js which manages petition when used as a widget.
4. Modified class.signature.php which is used by both of the above to write to the database table.
Here's some links to copies of the code as modified:
Code should you appear when you click on each of those. Copy and paste into an editor (eg:PSPad) should make it easier to read. I've commented in where the hacks are.
Haven't got round to modding csv.php yet, but browsing the database table via phpMyAdmin shows that the updates are going in just fine.
hope that helps,