YES, you can mark the time log as billable and add notes to that in the timesheet. Please refer below steps to do so;

1. Navigate to Left Menu > Time Log > Weekly Timesheet
2. Hover on the time slot that you want to edit and click  icon (Refer Image 1)
3. Put the NOTE, select  mark as the billable hour & click Save (Refer Image 2)

                                                                                         (Image 1)

                                                                                              (Image 2)