The data types of the column in your file connection for the CSV file AND Docuware field you're adssigning data to - the two must match. Most likely, your column in the File Connection is defined as text, and can't be matched to a date column of the File Cabinet. The option for assigning Field in the Fill With would not even appear if the data types do not match. This applies for dates and numeric data types.
So it looks like you need to adjust your File Connection definition for the CSV file to assign Type=Date instead of Text to the column..