Yes, you can do so by changing the Net Bill amount directly or writing up the time entries in the Invoices screen.
You can write up or write down the value of time entries from the Invoices screen while creating batch invoices. Else, you can change the Net Bill amount manually there, thus overriding any computed value and generating a write-up/write-down automatically. You can also write up or write down entries in the Time Entries screen itself by using its Batch Update screen.