Importing data into subform

Importing data into subform

Hi, 

I have a multi-line field that I want to replace with a subform in my deal module (so that we can date individual entries in the future). I have exported the content from the multi-line field that I will be retiring, but how do I mass update the subform to populate the info from the old field into the subform? Thanks in advance for any help! Hope this is not too confusing!