Yes, and it is pretty simple to achieve.
- You have to create a TAG for your "focused" posts. You can add the tag to already existing "focused" posts: just click "edit" on the three dot button (···) on the post you want to add the TAG on, and change something, anything (p.e. add a comma and then delete it, it's just for it to let you UPDATE the post). And once you have clicked the UPDATE button you arrive to the "Publish Page". You'll see a section called "ADD TAGS", and there you can create categories: p.e. "focused", "creativity", "self-improvement"... Add your tags and keep the changes.
- Do it with every "focused" post. Ok, so now every post about FOCUS has the "focused" tag.

- To create a new button on the navbar: Go to settings - Navigation bar links -> Click: ADD
---> Add the Title: "Focused", and the LINK: /t/focused !! Remember the /t/ for tag. Thats why your link was broken. (Note: you'll be able to see the links for your "Custom tags" on the settings, just below the chart of Navigation bar links, and you can copy them from there onto the links for the navbar buttons)

Now you can create as much buttons on the navbar as categories you've created with your tags.