Do you mean you want this new pose to have joint values of 0 for all joints? Or just to be defined as the default Home pose?
For the first, simply use the "Define as Zero position" button in the Kinematics Editor for any joint which requires it. Bear in mind this might cause the geomtery to jump around (depending on home you work) so please prepare a backup before you do, it might require some playing around to get it right.
For the latter, Open the Pose Editor for your robot while in modeling, adjust the Home pose and click "Update". This will give you a home pose with non-zero joint values.