Zotero to Roam Export
A Zotero addon for exporting to Roam's JSON format. Exports single or multiple items, as well as collections.
Download
Grab the xpi from the releases page, and install by choosing Install Add-on from File... from the cog menu in Zotero's Add-ons Manager.
Requires Zotero 5.0.
Instructions
Right-click an item, items, or collection, and choose Export to Roam. Save the export file, and upload into Roam using Import Files from the ... menu.
Thanks to @retorquere for help refactoring for Zotero's async architecture.