Claudine,
Timesheet App uses User's data entry profile mentioned in his 'User Profile'. If not found then it uses data entry profile 'ESS'. If it is one time, you can update the user entry profile here.
User profile also drives many other behavior like Approval, Worklist (suggestions), Target resources etc.
If you really want end users to have an option to select user profile, then you need to extend the timesheet app before the timesheet screen loads.
thanks
Krishna