How to add content source as a search refiner

Hi, we have 6 content sources in our search system and would like one of the refinement panel options to be able to filter these down by content source.

I understand that adding content source as a refiner in SharePoint 2013 is possible by setting the built-in ContentSource meta property to 'refinable', but how do I do this in SharePoint 2010?

Alternatively, I could make my own 'myContentSource' meta-property and refine on that but I don't know what the mappings would be.

We do NOT have FAST search.

Thanks for any help.