You'll need to rename the vendor's name to recreate him/her as an employee in QuickBooks Desktop (QBDT). Let me show you how.
I've attached a screenshot below, so you'll know what I'm referring to.
Once done, you'll be able to add the vendor as an employee. For further guidance, you can also check out this article: Set up a new employee in QBDT Payroll.
That should do it. Stay in touch if you have any questions. I'll be here to answer them. Have a good one!