When you connect to Datev, Payhawk will automatically create 2 custom fields for KOST1 and KOST2.
You can include all the values that you need by following these simple steps:
Go to Settings -> Expenses
Select Manage on the KOST1 or KOST2 values
In the KOST1 label place the user friendly name of the category
In the External ID place the value that you want exported into Datev