It is quite the opposite. If you want to use both portal and ASPX templates, you need to start with an ASPX master page (.master file). A detailed guide about how to create a shared ASPX master page for both portal and ASPX templates can be found in this documentation section - Using both ASPX and portal templates on a single site
One thing which keeps me curious is where exactly did you get the error saying the .master extension was not allowed? Was it in the Kentico administration interface and its "Page templates" application or in a Visual Studio when you were creating the physical file?