As n OPML file just contains the definition of which feeds you want to follow, not the actual RSS data. They’re handy when you want to switch from one RSS reader to another. You can usually export the settings from your old service and import them into the new.

Check with your RSS service but importing an OPML file should always add new services to the list, not overwrite.

I don’t think an OPML file tracks what you’ve read or not but I could be wrong about that.