okay, but the model 'hr_timesheet_sheet.sheet' already inherits 'mail.thread'. How can I add the 'mail.compose.message' so it inherits both models?

that is not an issue if you want to add the field just inherit the base model. You can either inherit mail compose or hr_timesheet. but the module you give in depends in descriptor file matters. The fields upto the inherited model will be available to you. Add the field you wish in your new model which is inherited from the base model. and adjust the position of the field in the view in xml file, for that you have to inherit the view also