Every column that gets indexed by the Search Service Application (SSA) has a Crawled Property created for it. When you go into the SSA via Central Administration and look at the Search Schema, you should be able to dig up the Crawled Property (usually prefixed with 'ows_').
You can then create a Managed Property--mapped to the Crawled Property--which will become available to use after the next crawl. You can then reference the Managed Property for use as a filter/refiner, or in a Display Template.
If your websites aren't on the same farm/sharing the same SSA, then you'll need to configure the SSA at the receiving end and ensure it's crawling your content.