Is there any way to homebrew actions to add to the "Actions in Combat" bar that appears right below Attacks on the Character sheet? I am homebrewing a feat to add the optional combat rules from the Dungeon Master's Guide (such as disarm or overrun) to my players' character sheets so they don't forget about the new actions they can take in combat. Right now I am resorting to simply leaving out a snippet so that it doesn't add too much space to the character sheet, but it would be awesome if I could just append them to that small list that already exists.

No. What you can do is make each as an action and just list them in the snippet. Whenever a snippet is tapped it activates the sidebar which contains the full description. So if the snippet just says “overrun,” when they tap it, if you listed the full thing in the description field they can still read it.

Sorry, I think that's what I meant, if I'm understanding you correctly. I was going to just make the separate actions without a snippet so it wouldn't take up more space unless you clicked on it for the full description. Unfortunately I tried that out, and it just looks terrible, so I added the snippets back in. I also made each one an option on the feat so I can select only the ones that are being allowed to show on the character sheet.

Ah, gotcha. Almost, what I meant was make the snippet either say something like “additional actions” or have each one just be the same list of actions (it looks slightly less stupid), but yeah basically. And the snippet for the feat would also just be that list as well, basically use the same for all of them so it at least looks uniform and intentional.