Can you dictate Emoji on iOS?
No, you can't.

Because there is 2 different keyboard sets, what can't be mixed in one dictation process. That could be the same like you'll try to dictate on 2 different languages in one time. That's hard enough.

But... It's just now, and who knows what kind of updates we will get with new iOS versions and new devices.

By the way. You can use some common text emotions on english keyboard. At least one:

:-)

All that you need to say, it's just: "Smiley".

Two more I know of the top of my head:

:-(    =   'frowny' 

;-)   =   'winky'