No, it's not possible to unmarshal such a document, because the document is invalid. For example:
<?xml version="1.0" ?>
<foo>
<bar><![CDATA[This should contain a nested cdata, which <![CDATA[Contains another CDATA]]>]]></bar>
</foo>
Fails to parse because the character sequence ]]>
can only be used to mark the end of a CDATA
section. In a nutshell, you can't nest CDATA
sections. You'll have to figure out another approach.
See the wikipedia article, and this question and answers for more information.