Hello Krista,
Unfortunately, I'm afraid there is no quick solution available for your situation. The problem is that to correctly "digest" the information present in an import file, computers (not just EPPI-Reviewer) need to have a predefined set of precise rules to "understand" what a given bit of data represents. CSV allow to subdivide data into the equivalent of Excel "cells", but EPPI-Reviewer can't know what kind of information (title, abstract, journal, auhtors, etc) is represented in each column, so can't in principle automatic digest custom-made CSV files. Our RIS-Export utility is designed to support a wide range of known formats used by many different online resources (including some CSV-based specific ones), but does not and will not be able to digest arbitrary CSV files.
Thus, I fear that the best solution for your case is to fall back to your original Zotero library: find the items you have already screened in there, export to RIS the ones that you have "included", avoid exporting the ones you have already excluded, and if there are any, export the ones you haven't screened already. Depending on how much work you've done already, I can see that this might be time consuming, but unfortunately I can't say we have already written a solution that would work for out of the box.
It is not in principle impossible to write a CSV import filter that will "understand" the default Zotero CSV export (I think!), but this won't be quick (will require a few days at the very least - assuming we can make room for this task immediately), so I'm afraid you are likely to be better off with a manual DIY solution.
I'm sorry we can't be more helpful,
Sergio