The easiest option is to compute the pressure directly with a compute and use it to replace the default pressure:
compute temp_null all temp
compute press_trasl all pressure temp_null
fix fNPH all nph/asphere x ${my_press} ${my_press} ${my_Pdamp}
fix_modify fNPH press press_trasl
Note that this topic has already been discussed in the forum and mailing list. You find a list of all the various contributions in this thread: Pressure for atom_style ellipsoid (probably a bug)