In Web TimeSheet its possible to enter time for a disabled user in 2 ways.

Option 1: Update an existing timesheet as an admin.
 
1. If the timesheets have already been generated, then go to historical timesheets select the appropriate user.
2. Edit the timesheet and make the required modifications and submit it again.
 
Option 2: Update timesheets as a substitute user.
 
1. Enable the user account. Remove the start or end date if the time that needs to be entered is beyond that.
2. Change the password, or go to advanced tab assign yourself as substitute user. 
3. Log out & log back in and you will see an option to login as a substitute to the disabled user
4. Login as the disabled user,  click on the required date and submit the timesheet for approval.