  We study inelastic (dynamical) impurity scattering effects in two-band
superconductors with the same ($s_{++}$ wave) or different ($s_\pm$ wave) sign
order parameters. We focus on the enhancement of the superconducting transition
temperature $T_{\rm c}$ by magnetic interband scattering with the interchange
of crystal-field singlet ground and multiplet excited states. Either the
$s_{++}$-wave or $s_\pm$-wave state is favored by the impurity-mediated
pairing, which depends on the magnetic and nonmagnetic scattering strengths
derived from the hybridization of the impurity states with the conduction
bands. The details are examined for the singlet-triplet configuration that is
suggestive of Pr impurities in the skutterudite superconductor
LaOs$_4$Sb$_{12}$.
