How can I test parts of my application against the output of third party application?

Question

I have a fairly new C# application, approximately 6 months old, that we are now trying to incorporate testing into. (Should have been done from the start but I can't change the past)

Parts of this application involve pushing some documents out to a folder. A third party application then picks these up, does its own work on them, and drops output files in another folder.

My application waits for this to be done, picks up the outputs, then continues its work.

This poses a problem when testing as I can't really install another version of the third party just for testing, nor can I mock it without ending up fully rewriting the third party application.

How can I test these parts of my application? (Preferably with NUnit)

Specifically, I'm asking about how to unit test my code against the output of the 3rd party tool. So it's a little bit different than another similar question, which involves isolating how code interfaces with a third party.