New Activity Field from Due Date

I am trying to create a new field called NextTaskDate which will be DueDate >= TODAY(), but I can't find the field Due Date in the New Custom Field section for Activities.

I want to add 3 fields to a Partner Portal List View for Leads, to show the NextTaskDate, NextTaskSubject & NextTaskAssignedTo.

I don't even know if this is possible.
Can anyone assist?

Cheers,
George