One way to do this is to create a different action of your own called something like Send Reminder. On that you have the same fields as you have with the reminder and add your custom checkbox in there. Then in the process replace all the regular reminders with this new one and add in the process string in some extra bits that send the standard reminder to the regular people and an extra reminder to the builder. I usually put conditions in that look at the check boxes and only send reminders if that checkbox is set rather than relying on an empty checkbox not causing a reminder to be sent.
In fact while I think about it, I hardly ever expose the raw actions that LDSD provide in any processes I produce. That way you can control what the end user or analyst sees and let the process take care of checking who should get which reminder.