Julie, you can create a profile for her with the name Cora Unknown, and later on if you discover the name she was born with, you can change it.
For the data field, you can estimate what her birth year might have been.
The more information you can add to the Biography section of your great grandfather and your grandparents, the more help others might be able to give you in identifying Cora's background.
For the Biography section of Cora Unknown, you might need to say, "we know nothing about her except that she married ____". In the Research Notes section, you can list the sources you have tried to look for information and been unsuccessful. All of this assumes -- correctly, eventually -- that someone else will try to help you and you want then to have all the information you have!