How to embed XML in <source>

What's the best way to have an XML fragment inside <source> or <target>
elements? Say my source is the following XML:

<myxml:string>File <myxml:accelerator
platform="windows">O</myxml:accelerator>pen</myxml:string>

I want the entire <string> element as the content of an <xliff:source>
element. Must I use one of the inline tags and transform my <string> element
into the appropriate form before I can embed it into <source>? If so which
is the best inline element to use and what would the the <source> contents
look like?

Why not let <source> and <target> support user-defined elements, so I could
just embed <myxml:string> without modifications?

Thanks,
Zartaj