There are a lot of "words" in Skritter which have more than 2 characters, e.g. names (3), chengyu (often 4), or expressions with 8, 10 or more characters.
Just add them to your list like you do with a normal word (the best is to use Chinese characters, not the pinyin) and if this expression doesn't exist in the Skritter database, it will ask you whether you want to add it. In this case, you have to add the definition and might be the pinyin (if it has characters with multi-pronunciation) as well as choose the correct traditional (if it has ambiguity). I normally use Pleco to get this additional information. Be careful with the tone change of 一 and 不, but your textbook should show this correctly.