Hello, @Michele30.

At this time, we can only assign a single class per employee. Assigning multiple classes to a single employee isn't available. Though you can change the class through the payroll settings as provided by IamjuViel above, the changes may affect to all of the payrolls for that specific employee.

For additional reference about classes, check out these articles:

• Create and manage classes in QuickBooks Online
• Create and assign employee classes

Let me know if you have any other questions. I'm a few clicks away to help. Have a good day!