  Implicit Monte Carlo (IMC) and Discrete Diffusion Monte Carlo (DDMC) are
methods used to stochastically solve the radiative transport and diffusion
equations, respectively. These methods combine into a hybrid
transport-diffusion method we refer to as IMC-DDMC. We explore a multigroup
IMC-DDMC scheme that, in DDMC, combines frequency groups with sufficient
optical thickness. We term this procedure "opacity regrouping". Opacity
regrouping has previously been applied to IMC-DDMC calculations for problems in
which the dependence of the opacity on frequency is monotonic. We generalize
opacity regrouping to non-contiguous groups and implement this in \supernu, a
code designed to do radiation transport in high-velocity outflows with
non-monotonic opacities. We find that regrouping of non-contiguous opacity
groups generally improves the speed of IMC-DDMC radiation transport. We present
an asymptotic analysis that informs the nature of the Doppler shift in DDMC
groups and summarize the derivation of the Gentile-Fleck factor for modified
IMC-DDMC. We test \supernu\ using numerical experiments including a
quasi-manufactured analytic solution, a simple ten-group problem, and the W7
problem for Type Ia supernovae. We find that the opacity regrouping is
necessary to make our IMC-DDMC implementation feasible for the W7 problem and
possibly Type Ia supernova simulations in general. We compare the bolometric
light curves and spectra produced by the \supernu\ and \phoenix\ radiation
transport codes for the W7 problem. The overall shape of the bolometric light
curves are in good agreement, as are the spectra and their evolution with time.
However, for the numerical specifications we considered, we find that the peak
luminosity of the light curve calculated using \supernu\ is $\sim$10% less than
that calculated using \phoenix.
