Specific Exchange rate type for Purchase order transactions

Hello,

Is there any transaction in customizing where we can default a specific exchange rate type to be used for specific transactions. To explain in more detail, I am setting up a new exchange rate type which will be maintained by Treasury. We want Purchase Orders (in a specific Purchasing Org) to be raised only with that rate type. There are other requirements as well like that Exhange rate field should be non editable in the Purchase order etc.

Any inputs would be of great help

Keyur