Thanks for getting back to this thread, @ktuva,
At the moment assigning labor time after processing Payroll isn't possible. You will need to add the time the moment you create the pay run so the paycheck will show the labor cost correctly.
If you're not using Direct Deposit, you can undo the payroll and run it again with the labor time on it. See the following article with the complete steps on how to add the time data: Get your project labor costs in real-time.
If you have any more questions about Projects or Payroll, let me know in the comment. I'd be delighted to share some insight with you. Have a nice day!