  We present an algorithm to extend subhalo merger trees in a low-resolution
dark-matter-only simulation by conditionally matching them to those in a
high-resolution simulation. The algorithm is general and can be applied to
simulation data with different resolutions using different target variables. We
instantiate the algorithm by a case in which trees from ELUCID, a constrained
simulation of $(500h^{-1}{\rm Mpc})^3$ volume of the local universe, are
extended by matching trees from TNGDark, a simulation with much higher
resolution. Our tests show that the extended trees are statistically equivalent
to the high-resolution trees in the joint distribution of subhalo quantities
and in important summary statistics relevant to modeling galaxy formation and
evolution in halos. The extended trees preserve certain information of
individual systems in the target simulation, including properties of resolved
satellite subhalos, and shapes and orientations of their host halos. With the
extension, subhalo merger trees in a cosmological scale simulation are
extrapolated to a mass resolution comparable to that in a higher-resolution
simulation carried out in a smaller volume, which can be used as the input for
(sub)halo-based models of galaxy formation. The source code of the algorithm,
and halo merger trees extended to a mass resolution of $\sim 2 \times 10^8
h^{-1}M_\odot$ in the entire ELUCID simulation, are available.
