OK, no one responded, so I was able to add a field and will share how I did it so if there's a better way, it can be pointed out:
1. I added a new em_tickets_ext extension table with the event_id, ext_id, etc.
2. Added my new extension class modeled from em-ticket.php and added logic to select, update and delete from my new table
3. added add_action('em_ticket_save','my_em_styles_ticket_save',1,2); to save to my new table.
4. added add_action('em_ticket','my_em_styles_ticket_load',1,2); which does a select on my new table to get my fields and add them to the $EM_Tickt object
5. Added a new input element for my new field to tickets-form.php