Using environment varibles in CPE Descriptor   

Hi!
I have a following Problem with CPE Descriptor: i have set environment variable PRJ_HOME, as my porject folder,
and I want to use it in my CPE Descriptor. In tag

<include href="${PRJ_HOME}/desc/reader/FileSystemReader.xml"/>

all is ok, but in Parameter InputDirectory it interprets it as String... and throws en exception
---- directory "${PRJ_HOME}/desc/data/input/text" does not exist

Have you some ideas about that?

<collectionIterator>
            <descriptor>
                <include href="${PRJ_HOME}/desc/reader/FileSystemReader.xml"/>
            </descriptor>
            <configurationParameterSettings>
                <nameValuePair>
                    <name>InputDirectory</name>
                    <value>
                        <string>${PRJ_HOME}/desc/data/input/text</string>
                    </value>
                </nameValuePair>