Actually I'm not sure if this is possible or not. I had setup a Community (new SF portal approach) and now I would like to share some of the Community tabs with my Force.com site. Btw, it is a "Force.com Community"
I found on the Sites documentation that this is possible using Apps:
Control the visibility of custom apps. If you want to expose a custom app and its associated tabs to public users, make only that app visible and make it the default to avoid exposing other pages. If any of your site pages use standard Salesforce headers, other visible applications may be seen by public users.
I tried to do this clicking on my site label and then on Public Access Settings, as a result I land on a profile called Customer Support Profile. Then on Apps section I click on Assigned Apps and the only listed App is Platform.
So I thought I had to assign the public site profile (Customer Support Profile) to the Plataform app. I went to Create > Apps, clicked on Platform, then Edit but Customer Support Profile is not listed !
I would appreciate advice here. Thanks !