Steemit could easily add Emoji as part of supporting Git Flavored Markdown - it would just be another plugin to their Markdown rendering engine. I wouldn't count it out for a future revision of Steemit.com, and derivative apps (like Busy, eSteem, and ChainBB) can add the support at any time before Steemit does (if they haven't already).
Then, you would include :smile: in your Markdown in order to have 
rendered in your text.