Managing dropdown content in regions...

I need a "Sales Rep" dropdown field in our "Retailers" region. The dropdown needs to be maintainable by the client (i.e. add/remove sales reps). This basically means we can't specify the sales reps in the template because it'll change over time.

Another similar scenario. Say we have a "Contacts" region. In this region there is a SLA field. How can we maintain the SLA dropdown so if it's changed (say a new gold level is added), any existing contacts could select the new gold level SLA?

How can we achieve this? I'm thinking a shared region but not sure how to incorporate it.

PS - if my question makes no sense ask for clarification :P