Pelo que entendi, pode ser definido como parte da política de grupo: (veja aqui)
In the left pane of the Group Policy Management Console, double-click User Configuration, and then double-click Administrative Templates (Classic Administrative Templates (ADM) in Windows Vista). Then follows the following steps to set and change the default saving file format in Office 2007 programs.
To change default file save options in Office Excel 2007
Double-click Microsoft Office Excel 2007, double-click Excel Options, and click Save. In the right pane, right-click Save Excel files as, and select Properties. In Save files in this format, select Enabled. In the drop-down box, select a default file save format. Click Apply to save the settings.