When using custom additional fields in Service / Incident Catalogs, is it possible to maintain the values of a Picklist field by updating data from an external system on a periodic basis via some script or API?

For example, if I want to add an Email Group field as a picklist (instead of letting requesters type in) and I want to populate this picklist with names of actual email distribution groups in Active Directory, is there a way to setup a background script on a schedule to keep updating the picklist with the values from AD?

Thanks,

Sourjya