Yes you can. You have to do two Things.
You find the Folder where the Layouts are under $JBOSSDEPLOY\jboss-portal.sar\portal-core.war\layouts
First copy one of the standard layouts to another folder in the layouts-subfolder for example "MyLayout". Inside you find two jsp files. One for the "normal" layout (index.jsp) and one for the layout when one portlet is maximized (maximized.jsp). You can edit and design this files with everything known from JSP, HTML and whatever you like.
To get this new layout to work, you go to the \WEB-INF Folder of jboss-core.war. There you find the portal-layouts.xml file. Open that, and copy the XML Structure of one standard layout, and replace the URLs to the files.
After that, and a restart of the Application Server, you can select the layout in the admin Portal.
I hope my description helps, it's realy easy, you will see.