Is there a way to add words/phrases not found in the dictionary (like slang/colloquialisms)? For example, I want to add "シュッとしてる” to the dictionary -- I've come across this phrase a lot, so it seems useful lol.
シュッと is in the dictionary already. If you can't find a word or phrase with the する part attached, try removing it.
To answer the actual question, Renshuu's dictionary data is from the JMDict project, so Renshuu doesn't control what words are available in the dictionary. If you want to suggest a new word, you have to suggest it to JMDict, and then if it gets added it will appear on Renshuu the next time the dictionary syncs.
Otherwise, you can make your own private words for your own use, that do not appear in the dictionary and are not verified in any way. To do this, just make a new list in Community Lists, search for your word, click the button to create a new entry, and fill in the information.
@gillianfaith AHH THANK YOU! I just tried putting in the word as a new entry, and it worked! すごく助かりました！
For anyone interested, シュッとしてる is slang for "looking sharp/cool/slender" often used to describe men -- but apparently (like all slang), there are variations in meaning. A little different from the シュッと that appears in the Renshuu dictionary. (This article was super interesting and goes into detail about how the meaning fluctuates with demography/context... for my fellow nerds!!)
