Nikki, what you're describing would be a custom field in Agile Central. There's actually an application in Agile Central called a Custom List so folks like me in the community may not immediately understand your question. Custom fields have to be defined by work item type so if you have a custom field called Customer that applies to stories and defects then you actually need two fields. The names can be the same- they need only be unique in the context of the work item type.
As to updating them from an external data source, I can't offer any suggestions there. There's no way in Agile Central to connect to an external data source for field definitions. I'd imagine that there's some way to programatically manage the values in a custom field through the WSAPI but that's outside my expertise and there's definitely no way from the UI to handle that except by a manual copy/paste exercise.
Hope that helps.