It probably depends on where the emoji you want to use comes from, but it will undoubtedly end up being uploaded as an image, and the system takes over what happens to all images at that point.
Here's a site,
Emoji Copy-Paste, you can just click on what you want and that action puts it in your clipboard, then just hit Ctrl-V to paste it into the text box and will look like this (this is actually the most user-friendly option I've found):
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But most times you'll right click to copy an image, paste it here and the system automatically enlarges it, plus any animated .gif image would be lost, background transparency would also be lost, so it would look (horrid and annoying) like this:
Even if you have the direct url to the image and use the [ img ] bbc code, it still ends up the same way, enlarged as a .jpg. If you give us an example of what emoji or where it would come from, might be able to give a better answer, but it's not very likely it'll work here with how the site handles images.
