This article explains what to do if you hire an employee who doesn't have a P45.
First, follow the steps outlined in this helpful guide from HMRC. The second point details exactly what to do when your new employee has no P45.
Next, when you do eventually receive the employee's P45, you'll need to edit the payroll profile for that employee.
Make sure to enter the employee's updated tax code, as well as the total pay and tax in previous employment for this tax year. When you submit the following month's payroll RTI, the year-to-date figures will automatically be recalculated.