There is always a requirement from customers asking for adding custom attributes in Websphere Portal server. Along with my colleague, I have written an article in Portal Lotus WIKI where we clearly mentioned on how to add the Custom attributes in Portal Server.
Removing Custom Attributes
For removing any of the custom attributes you need to follow a manual way of editing the wimextensions xml file (present in <portal or dmgr profile>/config/cells/wim/model directory). As listed in Infocenter, please edit the XML file and then Restart Portal to view the updated changes.