You can enter timesheets in both PlanRight and WFM for both sync modes. However, in order to sync timesheets correctly, the task data should be mirrored across both systems.
For our Plan in PlanRight sync mode if there is a timesheet registered under a task in WFM, an instance of the same task needs to be assigned on the PlanRight schedule. This task instance should also be assigned to the same staff in PlanRight as in WFM (e.g. whoever the timesheet was created for).
In the Plan in WFM/XPM sync mode PlanRight will automatically populate your WFM task data onto the schedule. So in this case the tasks and timesheets are already mirrored.