Registration Form Modification for PHP Site

I have a site up purchased from Wroupon. The site is a fundraising site and as people register I need them to be able to identify the cause they are supporting. I have already hard coded a simple drop down with limited options, but would like to be able to add to this list either through just adding to a database or by adding to a json array. Each organazation has different sub-catagories that although are repeated will be specific to that organization. I would also like to add this functionallity to our facebook connect registration. Please respond and I will send you a link to my site and go further into the project.