The ability to add your very own custom emotes on Twitch is reserved for an upgraded account to an affiliate or partner channel but also verified accounts.  Once verified you can add these emotes to your heart's content, within limits, of course.
Twitch affiliates start with one emote while a partnered account can upload six after the account has been promoted. Increasing the amount of emotes then depends on the number of subscriber points the specific channel earns.
Affiliates can get up to five Twitch emotes, whereas partners can increase their slots to 50.