You can add it later via Words, Grammar, or Add Your Content.
There isn’t currently a way to add words in either the Words or Grammar sets once they’ve been marked as skipped or known.
However, if you later decide you want to learn one of those words, you can search for it in the Words tile and add it manually.
If you prefer to learn it in context, you can add it as a sentence card through the Grammar tile or by using the Add Your Content tile.