Associate the Newsletter with Appointments registered user.

What I am trying to accomplish here is to assign an user ID to every appointment so I can easily create reports and track my appointments users with out having them have to sign in. Is there an easy way to write an add on or MU plugin to have the appointment confirmation box info (name and email) sign them up for a newsletter (called appointments)? Then instead of having uses for appointments plugin requiring them to be a WP user, they will instead just me signing up for a newsletter which associates them with the same user id each time they sign in. I guess you would then have to disable the sign up existing member function and code it so that the email either signs them up for the first time or logs them in again.

Am I making any sense? Do you think a developer could easily do this or might there be an even easier way to recommend to a developer to go about this?

thanks,
matt