class CustomEverywhere(xpsi.Everywhere):
    """ Custom surface radiation field globally spanning the surface. """
    
    @classmethod
    def create_parameters(cls, bounds, values={}, *args, **kwargs): 
            ......
             phase_shift = Parameter('phase_shift',
                                      strict_bounds = (-np.infty, np.infty),
                                      bounds = bounds.get('phase_shift',None),
                                      doc = "The phase of the everywhere [cycles]",
                                      symbol = r'$\phi$',
                                      value = values.get('phase_shift',None))
              
              return cls(*args,
                         custom=[...,
                                 phase_shift],
                         **kwargs)

values = dict(phase_shift = 0.0)
everywhere = CustomEverywhere.create_parameters(bounds=...,
                                                values=values,
                                                ...)