When you create the data source in Tableau, you can change the data type of the column under "Edit Data Source". Change it from numeric to string. It should preserve the formatting of the data you have in those cells. And the field will become a string field. You can make another calc that does if ISNULL and set it to whatever you want, ELSE keep the value as is.
Thank you very much Joe. I hadn't thought about manipulating the dataset within Tableau.