Hi there, @Salynn.
Thanks for taking the time to share your first post in the Community. I'm here to share information on adding employees' hire dates in QuickBooks Online Payroll (QBOP).
Since these are terminated employees, I'd suggest you look up each of your employee's records. You can also base it on their first paychecks.
Here's an article to guide you on setting up a prior payroll for QuickBooks Online Payroll.
Also, ensure to tagged these employees as terminated or not on payroll status to ensure they're no longer appear on your active payroll list. check out this article for your future reference: Terminate or change employee status payroll.
Visit and post more questions in the Community, @Salynn. I'd be delighted to assist you again.