While importing Test Cases into VSTS from another test management system for a customer, I needed to add a multi-line text field to Test Cases. VSTS has supported adding custom multi-line text fields for almost a year now and this seems to work for every Work Item Type except Test Cases. When attempting to add the field, Text (multiple line) is not present in the Types drop down list.
According to Microsoft, this seems to be a bug and there should not be any reason to block adding a multi-line text field to Test Cases. As a workaround, you can temporarily add a new multi-line text field to another Work Item Type and then add the existing field to the Test Case work item type.