Vista - LINQ to XML Add Nodes?

I have an XML document that looks like this

<xml>
<item>
<stuff />
</item>
<item />
<item>
<stuff>
<otherstuff />
<item>
</stuff>
</item>
</xml>

And I need to select each of the top-level <item> elements and add
this

<subitem />

What's the fastest way to do that with LINQ to XML? I don't want any
<item> less than 1 level deep. I could do this with XPath but am
trying to get into some LINQ/XML goodness. I'm also working with an
instance Class, so doing anything fancy with static Types is not
possible.

Thanks.