OPML Files are usually at the core of RSS Feed Aggregators and if you were using an RSS Feed Reader before Elytra, chances are, it allows you to export your feeds as an OPML file which Elytra can’t import.
In the iOS App, you can navigate to Settings > Import/Export OPML and then tap on import to begin importing your OPML file.
On the Web App, you can navigate to Settings > Import/Export Feeds and then tap on import to begin the process.
Elytra also allows you to export all your feeds (including Folder structures if you have any) into an OPML file.
If you have existing Feeds and Folders, and import a new OPML file, no existing feeds will be removed. If a folder with the same name exists, it will be reused.