How to implement "import OPML" and "export OPML" in a podcast app?
Use POST /podcasts. You can pass up to 10 rss urls in the "rsses" parameter, and you'll get podcasts from the response.
You can also batch import Apple Podcast ids (iTunes ids), Spotify ids, and Listen Notes ids.
By using Listen API, you get podcast names and RSS urls from the response. and it should be straightforward to generate an OPML file.
If you haven't used Listen Notes Podcast API yet, get API keys here - Get API keys