Have you tried Ultimate Launch? It won't make a new tab, but you can set the parameters of UL to be only the height of HTC home. Then you just 'slide' your finger right to left or left to right, and you can switch between screens. That way you set htc home as one screen, and calendar (I strongly recommend today agenda) as another screen. You can put more screens for whatever you want, then you just 'slide' between them. This will also give you more space to view your appointments. IMHO, this is a better option that integrating a new tab into htc home.
UL Forum:
http://forum.ppcgeeks.com/showthread.php?t=19474
Today Agenda:
http://tiews.info/index.php?show=sof...agenda&lang=en