How to create a lookup field after the two forms are already established?

How to create a lookup field after the two forms are already established?

Hi.

I have imported Excel data to create one form. I would like to import data for a second form, where one data field contains data which is identical to a field in form one. Is it possible to create a lookup field after I establish the second form, with its data?

Also, I see that there is an Excel Migration tool for Windows users. Is there something comparable for Mac users?

Thank you.