Hello Zoé,
Please don't worry about threads, the bulk of our support requests arrive via email and I'm not sure if many people actually browse the forum to see if their questions have already been answered.
The RIS import native in EPPI-Reviewer can include one (and only one) "imported ID", by looking in these two fields from an RIS file:
U1 -
or
ID -
I would recommend to use the "ID -" field and only that one. I confess I'm not familiar with Zotero, not enough to know how to customise its own "export to RIS" feature - I hope this will be possible for you.
The above will work, but might still not be the perfect solution for you, depending on your specific needs. The catch in question is that importing items will generate new "EPPI-Reviewer" IDs, so when using the "Export to RIS" feature of EPPI-Reviewer these IDs will be dutifully exported. Specifically, they will be exported using the "U1 -" field/tag, while the value of the "Imported ID" will be exported in the "U2 -" field (if any).
I mention this because it's likely that doing a round trip, Zotero to EPPI-Reviewer to Zotero will most likely lose the original IDs as new ones get generated... I hope I've managed to explain what I have in mind, please do ask for clarifications, if I didn't.