API connectors now have the ability to have custom mappings from the SIS to the generated OneRoster CSVs. To enable this, check the box in the settings for 'Custom Configuration for Connector'. Click 'Configure' to set up the mappings.
In the configure menu, you will see your SIS's endpoints, and what they will map to in OneRoster (if applicable) under 'Collections'. On the right, you will see the fields that can be mapped. Here you can either type the field from your SIS you wish to pull from, or choose an option from the dropdown list. The options in the dropdown depend on the SIS you have chosen. When you have finished making the mappings, be sure to click 'Save'.
Aside from the standard OneRoster fields, the custom config will also allow you to create new metadata fields. Click the '+' icon next to 'Custom Fields' to add a new metadata field. As with the standard fields, you can either type the field you wish to use, or select from a dropdown.