I have "liferea feed reader" on Linux for RSS feeds. Is there a way I could import all my added feeds there to Android? (NOTE: the extension of RSS exported file is "opml".)
Android doesn't have a built-in RSS feed reader, but there are many RSS feed reader apps (and websites) that will happily import an OPML file.
If you want to use a website, or an app with a website-synced back end, then Google Reader is very popular, and you can definitely import OPML files into Google Reader. On your device you can then either visit the Google Reader webpage, or use the Google Reader app, or one of the other many Android apps that sync with Google Reader.
To import an OPML file into Google Reader, click the cog/gear icon in the top-right of the page, then click Reader Settings and then Import/Export. You should then see the option to upload your OPML file.
RssDemon imports OPML files.
JustReader says it does, but I couldn't find it and have asked the dev about it.