If I understand your issue correctly, this is what I would do …
Download the entire list of subscribers you wish to sort
Create a list named – Without Names in the newsletter app
Open your list with a spreadsheet and sort the list the way you want – Delete all the contacts with names
Upload them back up into the new list you created – make sure you select update
Now you can send the contacts with no name the letter asking them to fill out the form with their name and email and it will update their contact information