In order to (mass) import data into a Form Reference Field you will have to reference the form number of the form in the process you want to reference.
In the example as shown below. The field Process Form Reference From Opportunity is a Process Form Reference field from the Opportunity process in the Order process. If you are planning on importing the order by ways of an excel import as per the example your spreadsheet should look like the screenshot below.
Number: is the order number you want to update
Process Form Reference From Opportunity: is the form you are referencing in the Opportunity process.