Custom CC Processor Parameters

I'm creating a custom CC processor for iMIS that will be used by Chapter administrators. The gateway will be the same for all chapters but each chapter will have their own account. Is there a definitive list of the value names passed in the name/value string? Can the string be manipulated based on settings in iMIS? For example, can I add "ORG=[selected org name]" to the value string based on the currently selected or logged in user, or are all the parameters fixed?