Items By Group added to custom post type

Ok, not real sure how to explain this. But i have an existing custom post type with some custom fields that is working fine for detailed information about an event. what i am trying to do now is to add items from a pre-existing table to a certain category for this event.

like,
event1 has details
event one has three categories associated with it – FOOD, MUSIC, CRAFTS
i have a table that holds say vendors in it (add, edit delete already working).
i need to add a ajax type area to the admin side of the custom post type to be able to add any vendor to one of the categories.

so if i went to the event custom post type and wanted to add McDonalds to the food category for this weekends event, i would just use a select box and add it. I could also add Ozzy Osbourne to the Music category for this event. I could also add Wendys to the FOOD category.

hopefully this makes since.

thanks in advance for any assistance.