How to add a contact ID to contacts I'm importing with migration

How to add a contact ID to contacts I'm importing with migration

New user, business owner here, not a data specialist.
I'm attempting to migrate import data from my old CRM, Daylite, via a .csv file. I feel like I've mapped the columns in the .csv file, however when I click save and next, I get a message saying the mandatory Contact ID field needs to be mapped.  My old CRM didn't use a Contact ID, so I'm not sure of the best way to move forward here. I have some 6,000 contacts that need to be split up to import (you can't migrate that many contacts in in one go), so trying to do that with a spreadsheet seems complicated. Can I do it within Zoho? Suggestions appreciated.
Thanks!