In order to (mass) import data into a Form Reference Field you will have to reference the form Number from 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 into reference fields via excel import, your spreadsheet should look like the screenshot below. 

Number: is the form number you want to update. In this case, the Order number.

Process Form Reference From Opportunity: is the form Number you are referencing in the Opportunity process.