This topic has been bumped,so I give everyone a little update. This is doable since may 2020
https://github.com/discourse/discourse/commit/0996c3b7b36c37c3b478985da6e968f54dc4835c
We can do this in the /admin/customize/emojis page
I prefer to use a plugin to sort everything out and it works perfectly : GitHub - MonDiscourse/iunctis-emojis · GitHub