  We calculate open charm production in $Pb+Pb$ reactions at SPS energies
within the HSD transport approach - which is based on string, quark, diquark
($q, \bar{q}, qq, \bar{q}\bar{q}$) and hadronic degrees of freedom - including
the production of open charm pairs from secondary 'meson'-'baryon' (or
quark-diquark and antiquark-diquark) collisions. It is argued that at collision
energies close to the $c\bar{c}$ pair threshold the dominant production
mechanism is related to the two body (or quasi two body) reactions $\pi N \to
\bar{D} (\bar{D^*}) \Lambda_c, (\Sigma_c)$. Estimates within the framework of
the Quark-Gluon String model suggest cross sections of a few $\mu b$ for $\pi N
\to \bar{D} \Lambda_c$ in the region of 1 GeV above threshold. The dynamical
transport calculations for $Pb + Pb$ at 160 A$\cdot$GeV indicate that the open
charm enhancement reported by the NA50 Collaboration might be due to such
secondary reaction mechanisms.
