  A charged Higgs pair can be produced at an ee collider through a t-channel
exchange of a heavy neutrino (N) via e^+ e^- -> H^+ H^- and, if N is a Majorana
particle, also via the lepton number violating (LNV) like-sign reaction e^\pm
e^\pm \to H^\pm H^\pm. Assuming no a-priori relation between the effective
eNH^+ coupling (\xi) and light neutrino masses, we show that this interaction
vertex can give a striking enhancement to these charged Higgs pair production
processes. In particular, the LNV H^-H^- signal can probe N at the ILC in the
mass range 100 GeV < m_N < 10^4 TeV and with the effective mixing angle, \xi,
in the range 10^{-4} < \xi^2 < 10^{-8} - well within its perturbative unitarity
bound and the neutrinoless double beta decay (\beta\beta_{0\nu}) limit. The
lepton number conserving (LNC) e^+ e^- \to H^+ H^- mode can be sensitive to,
e.g., an O(10) TeV heavy Majorana neutrino at a 500 GeV International Linear
Collider (ILC), if \xi^2 > 0.001.
