This question was early asked at github long time ago without answer.

In Google Chrome it's possible to hint user to use some extension from Chrome Store by placing file extension_id.json with content

{
 "external_update_url": "https://clients2.google.com/service/update2/crx"
}

to /usr/share/google-chrome/extensions/.

When user opens browser, extension will be auto installed from Chrome Store if it was not previously installed and then removed by user.

Is it possible in Opera?