Background: RTC 2.0.0.2

I need to populate the "Priority" field depending on the work item type (e.g. Critical, High, Medium, Low for Defects, and just High, Medium, Low for another work item type).
Of course, I would not like to split the Priority field definition just for that, as it would also affect other areas (e.g. Work Items by priority report).
I thought about using value providers, more specific the value set providers to filter the priority enumeration. But based on documentation, it seems you can filter enumerations based on other enumeration fields, and I do not know if this is the case.
So my questions are:

1- Is the "type" built-in field a kind of "enumeration"? If so, how can I know the id and the literals?
2- If the "type" built-in field is not an enumeration, is there any way to accomplish what I need?

Thanks in advance,
-- Claudia Callegari