How can I use loop in graphical message mapping

The source input XML is like:

<bookstore>

<book>

<item1>xxx</item1>

<item2>yyy</item2>

</book>

<book>

<item1>zzz</item1>

<item2>www</item2>

</book>

........

</bookstore>

I want to create an output XML like

<output>

<item>

<item1>xxx</item1>

<item2>yyy</item2>

</item>

<item>

<item1>xxx</item1>

<item2>www</item2>

</item>

<item>

<item1>zzz</item1>

<item2>www</item2>

</item>

<item>

<item1>xxx</item1>

<item2>yyy</item2>

</item>

.....

</output>

That means I want to do permutation and combination for all <item1> <item2> under <book>.

Of course I can write a customer function to do it, but I want to basically use the buildin functions to do the graphical message mapping for this.

Any idea? thank you.