The easiest way to create an ADMX template is to build an ADM template first and then convert the latter with the help of Microsoft’s free ADMX Migrator tool.
Administrative templates let us create custom Group Policy settings. Administrative template files have two different versions. Windows Vista introduced templates with the .ADMX extension. These templates use an XML syntax and can be a lot more difficult to decipher and create by hand. On the other hand, templates with the .ADM extension are straightforward and have a simple syntax that allows you to create new Group Policy templates quickly.