Thanks for checking this with the Community. I can share some information about adding categories automatically.
Being able to auto-fill categories when selecting an employee/payee on your expense transactions is currently unavailable in QuickBooks Online. As a workaround, you can create an Unscheduled Recurring Expense transaction for your employee/vendors. This type is useful for transactions that contains a lot of detail that you don't want to retype, but that aren't needed on any set schedule.
Here's how you can create a recurring transaction:
Click the Gear icon.
Under Lists, select Recurring Transactions.
Select New button.
On the Transaction Type drop-down, select the transaction you'd like to create.