Yes, you can add any charges at the time service entry sheet (ML81N)
Do the settings on ML90, then in service entry sheet, click on the condition tab.
Here, you can add extra charges as per your service pricing procedure - calculation schema.
And when you accept the service, then your accounting entry will be as per new net price.
But you can't see the add charges amount in PO-service tab. You can see the net price in PO history tab.