kayoo123 the underlying emoji picker is using the list of emojis from EmojiOne, most likely based on this file at some point https://github.com/emojione/emojione/blob/master/emoji.json . The same list is used by the post formatter of Flarum to render emojis in posts.
So... short answer, no, it's not possible to add other emojis. If some EmojiOne emojis are not showing up in the picker or in the posts, it's possible they use an outdated version of the emoji list.
Technically it would be possible to dynamically push new emojis into the EmojiOne list, but it would require altering the post rendering as well and remove the use of an external CDN, and this is out of the scope of this extension.
