Its only about a TB but if your saying i cant do it bit by bit i wont. Ill imapsync users email and create accounts as needed
If you don't migrate your users & settings properly in one go you'll be back here taking up loads more of our time trying to fix the mess you have created against best advice.
Quite frankly you should have done this a long time ago.
The script essentially makes a standard SME backup less emails & files making it much quicker to migrate your server.
You can sync those files over later at your leisure.
So please, do the right thing, do it once, do it now, and save everyone a lot of time.