XML editing possible?

I would like to use the FCK to edit XML document's text content, the structure (that is not to be modified) is quite similar to (but not exactly) XHTML. This almost works now, but unfortunately the editor changes the document structure, resulting in errors when validating the uploaded document in the server side.

Is it possible to configure the editor not to touch the structure at all? Would it be difficult to modify the source code to achieve this?

Thanks,

Tero