Unlike Sony Ericsson, HTC has never released a SDK to allow developers to add their own tabs to TouchFlo3D. This is a shame, as TF3D covers up the whole of the today screen, removing one of the major customizability features of Windows Mobile.
Of course the developers at XDA-Developers are never ones to let a simple thing like a missing SDK stand in their way. TouchFlo3D has now been fully reverse engineered, and now the first custom tab has been produced for the interface by D-MAN66. The app, simply called Calender V.05, ads a much missed Calender tab to TF3D. It is still in early development, and installation is still complicated, but it appears to work pretty well.
Read more about this new development in this XDA-Developers thread.