Hi,
Thanks for using our plugins. I can try to help out here, however, I am limited in supporting the features as is and limited in supporting customizations.
I looked through the coding and it would not be possible at this time to add a field without modifying the Event Tickets Plugin.
You can add the field:
plugins/event-tickets/src/views/tickets/rsvp.php
After you move it to your theme following our themers guide.
However, there is no hook to add that field when creating the attendee. So you would have to modify the plugin, but those updates would be lost on each update of it.
If you think that is something you would like as a feature I encourage you to visit our User Voice Page and either upvote an existing request or make a new request there.
http://tribe.uservoice.com/
Cheers