Rayyan states it is compatible with the following text formats: EndNote (.enw), RIS, CSV and PubMed XML.
While you can export records from most databases as .RIS files and import them into Rayyan with ease, I strongly recommend you upload your files into a citation manager such as Refworks or Endnote first.
Benefits of taking this extra step include:
A back up copy of all your files located in a separate location.
Access to the citation manager's other useful tools such as citation page builder, write and cite et cetera.
Uploading records to an existing review
1) Open up the review you wish to add records to.
2) On the left, where it says 'search methods' select 'add new'.
3) Select the additional files you wish to upload and hit 'continue'.