  The formation of a $2.7\ \rm M_{\odot}$ supermassive neutron star is
explored, as the possible companion of PSR J0514--4002E. Magnetars may
experience super-Eddington accretion. Observationally they may manifest
themselves as ultraluminous X-ray pulsars. We propose that supermassive neutron
stars may be formed through ultraluminous X-ray pulsar phase, if the
ultraluminous X-ray pulsar phase can last for $10^{5}$--$10^6 \ \rm yr$. The
accreted material will also bury the magnetic field of the neutron star.
Assuming accretion equilibrium, the final output may be a millisecond
supermassive neutron star. In order for the ultraluminous X-ray pulsar phase to
last long enough, a magnetic field configuration of the low magnetic field
magnetar is required. The mass, magnetic field and rotational evolution of
super-Eddington accreting neutron stars are rather robust against different
assumptions, although many of the model details are yet to be determined.
