Why timesheets are linked to a User instead to an Employee ?
As per subject timesheets are User based so you should create a system user foreach timesheed you want to manage. Really timesheet are prepared starting fro Employee form but records are stored using user_id.
In fact if you try to prepare a timesheet for an employee who is not linked to a system user openerp reply with "employee should be linked to a user" and you can't go on...
thanks in advance for some answer rgds