I work for an institution where we take a lot of photos for archival purposes, and tag them with the photographer's name (using the Author tag), the subjects of the photo (using the Description tag), the date, etc. Is there any way (native to Facebook or third-party) that I can have Facebook parse these existing tags to auto-create descriptions of these photos when I upload them to Facebook?
There is no native way to do this.
You will need to look up a third party application that reads metadata for the specific purpose of uploading it to Facebook.
Otherwise you need to use the API and run a script yourself https://developers.facebook.com/docs/graph-api/reference/user/photos/#Creating
(Programming and app recommendations are out of scope for this site)