I have answered this question before - provided a lengthy tutorial for it.
You can only utilise custom icons if you’re creating a custom leaderboard. If you aren’t making a custom leaderboard, you will be unable to change any icons.
This being said, you could fork the CoreScript responsible for the leaderboard. Put its contents into a LocalScript in your game, then modify it as you wish. Keep in mind that forking means that you won’t receive updates from the original branch, however you’ll be able to essentially reproduce the leaderboard with your own changes added.
The repository for Roblox CoreScripts is available here: GitHub - Roblox/Core-Scripts: All of ROBLOX's core client scripts.
You will have to edit a lot of it’s functionality, as LocalScripts do not run at the same context level as a CoreScript. An example is sending friend requests.