extend xml output of FosRestBundle to desired (and actual used) format

How can I adjust the output xml of FosRestBundle?

For the reason that I have to jump on an existing api, I need to built xml output as desired and actual used.

So, for example, rest give me

<result>
    <entry>
        <name>testname</name>
    </entry>
</result>

and I need to get for example

<Authentication>
    <name>testname</name>
</Authentication>

How can I handle this? Any one got some informations or a complete example?

Thx and regards