If you have added new fields to a sheet generated from a subtable, or have added a reference subtable to a sheet and would like to add new fields from the parent sheet, you can use the link fields from parent sheets option to link the new fields.
To use this type of linking, navigate to the Design Mode through the Change Design button. Click on Form Tools. You'll see the available parent sheets as options listed under Link fields from parent sheets, in the Sheet Linking Tools category.
When you click on the sheet name, you can drag and drop the available fields listed in the pop-up window.
Don't forget to save your design before exiting the design mode.