I figured it out in case anyone else is having this issue. You can add fields to the main data source by clicking on Design Tasks > Data Source. Select your main data source and right click "myFields" and select "Add...". You can add a group and you can add all of your custom fields to that group and you can perform data validation, lookups, and rules on them just like you'd be able to do if the main data source was not a web service.