If it is Portal 7.0, then then all administration of portal including desktop administration like adding subtabs etc is done through psconsole http://machinename:<port>/psconsole
1. Login to psconsole -> click on portals tab
2. Click on Add Dn button and add the org/user/Role for which you want to add the subtab
3. After adding, from Select DN listbox, select the respective org/user/Role
4. click on Manage Containers and channels link
5. From view type click on JSPTabContainer
6. Then from right hand frame, you can add a new container may MyFrontPageTabPanelContainer which sits on JSPTabPanelContainer
7. Once you do this, you can add channels to this MyFrontPageTabPanelContainer