Dropdown values for Service blueprint form

I have a dropdown in a service blueprint form which I want to populate from external source. I have figured out creating and binding custom action to accomplish this. This works fine as long as the custom action return an array for strings. But, I would like to return array of label and value pairs (e.g.: value is id & label is name) and use value when dropdown value is selected. It looks like form designer supports this for static values. Do you know how to accomplish this using custom action? Thanks for your help.