Is there a way to convert Captivate 8 files to Captivate 7?
No. Downgrading a Captivate project to a previous major version has never been possible.
And since Captivate 8 is now several versions behind, you are entering territory where upgrading in one step to the current version might soon become impossible.
I recommend you think about updating your Captivate version and getting all courses ready to be HTML5.
Moreover there was a major change in UI and functionality from 7 to 8, where responsive projects with Breakpoints were introduced.