I already answred your support thread and for your convenience and for others who might encounter the same problem I pasted my reply below:
Indeed, you are corrcet in your observations and your scenario cannot be achieved with the current implementation of RadFormDecorator. The reason is that the RadFormDecorator control does not skin elements that have a CSSClass applied, because the script automatically assumes that when an element has a class name, it is already skinned.
You can try playing with the DecorationZoneID property of the control - it allows skinning only of particular zones of the page.
the Telerik team