Javier,
this will be possible in Plesk 11.6
You can find the release notes at:
http://download1.parallels.com/Plesk/PP11/11.6/release-notes/parallels-plesk-panel-11.6-for-linux-based-os.html
Edit: It is not possible to do this in the current Plesk version, unless you are a Plesk Expert or...