Emoji are just images. You can upload whatever images you want, within size limits if they are to be hosted on the NGDF server. If you host them elsewhere, there aren't any size limits (but linking really outrageous size images may annoy a moderator enough to take action).
So here is an emoji that is not in the current NGDF repertoire:
That one is just a .jpg image uploaded like any other image / photo.
If you were to use that image regularly, it might be appropriate to
shrink it to a size similar to the native NGDF emoji.
.
- Graham