Append a new schema in XHTML

[condo4]

Hello,
I need to append some elements to XHTML
I need to write the schema for an XML like this :
_________________________________________________
<?xml version="1.0" encoding="utf-8"?>
<root xmlns="?????" xmlns:my="????" schemalocation="toto">
<p>toto titi tutu <br />titi toto tata<br /></p>

<my:pictures_frame>
<my:picture file="t1.jpg" title="desc 1" legend='no' />
</my:pictures_frame>

<p>popo yuyu yaya</p>
<ul>
<li>gfsdqgfasd</li>
<li>greqz</li>
</ul>
</root>
__________________________________________________ __

This file will be transform with an XSLT process into a valide XHTML
file.
And it will be included in a page (with html, head, body...)

I need a schema with:

- to have a root element equivalent to a DIV element
- to add new elements in another namespace define in another schema or
in this new schema.

I try many things (I saw some examples on W3C website) but nothings is
valid.
I work with Eclipse and the php plugin (with XML).

If somebody can help me.....

Thanks