Can I manually migrate comments? I have a Drupal 7 site and I'm thinking about upgrading it to Drupal 8. I want to refactor the site, so I just need to export the content.

So far, I've tested exporting content types using feeds module and a csv file, as I don't quite understand how to do the export using .yaml files. It did work: I exported the stuff I wanted using views in my current site, and then imported it back using Feeds.

I would like to know if there's a way of manually importing into Drupal 8 all the comments (where I already have imported the content).

I will appreciate any suggestion! Thank you!
Give a try to Migrate module that is for Drupal 7 and comes as part of Drupal 8. It can import comments. You might also be interested in reviewing all migrate tools listed on https://www.drupal.org/docs/8/upgrade/drupal-8-migrate-modules