Hello
I have to change "unknown" contents of XML. The structure and content itself is valid. 
Original  
<blabla foo="bar">
    <aa>asas</aa>
    <ff>
    <cc>
            <dd />
    </cc>
    </ff>
    <gg attr2="2">
    </gg>
    ...
    ...
</blabla>
becomes
<blabla foo="bar">
    <magic>
        <aa>asas</aa>
        <ff>
        <cc>
            <dd />
        </cc>
        </ff>
        <gg attr2="2">
        </gg>
        ...
        ...
    </magic>
</blabla>
thus, adding a child straight under document root node (document.documentElement) and "pushing" the "original" children under that. Here it has to be done in plain javascript (ecmascript).
The idea now is to
// Get the root node
RootNode = mymagicdoc.documentElement;
// Create new magic element (that will contain contents of original root node)
var magicContainer = mymagicdoc.createElement("magic");
// Copy all root node children (and their sub tree - deep copy) to magic node
/* ????? here
    RootNodeClone = RootNode.cloneNode(true);
    RootNodeClone.childNodes......
*/
// Remove all children from root node
while(RootNode.hasChildNodes()) RootNode.removeChild(RootNode.firstChild);
// Now when root node is empty add the magicContainer
// node in it that contains all the children of original root node
RootNode.appendChild(magicContainer);
How to do that /* */ step? Or maybe someone has a much better solution in general for achieving the desirable result?
Thank you in advance!
Answer: The solution by maerics works perfectly.