Custom fields in existing Sitefinity modules

Hi all,

I'm trying to add a custom field to the Events module to allow users to select a Sales Person an event should be associated with. The 'Sales Person' field needs to come from a pre-populated list out of our CRM, so what I'm trying to do is create an ajax call to the CRM to obtain a list of sales people, return that to Sitefinity as JSON data and then populate a drop-down list with which the person creating the event can then chose the relevant Sales Person to associate the event to.

What's the best/easiest way to achieve this?

Thanks,
Phil.