How to add a new field to an existing form

How to add a new field to an existing form

Let's say that I have a form with a dozen fields and thousands of records. How can I add a new field to this form and import the new data into it? I have the data in an Excel file.

The only way I can see to do this is to import the XLS and just make a whole new form, but this will break all the relations I've made with the original form.

This seems like a simple and fundamental thing to do, but I'm not seeing it. Thanks.