The following work

\uchar{32}{19} (for the -- ndash)

\def\ndash{\uchar{32}{19}\autoinsertnextspace}
 
\uchar{32}{20} (for the --- mdash)

\def\mdash{\uchar{32}{20}\autoinsertnextspace}
 
but it would be preferable to be able to use -- and --- instead of \ndash and \mdash.

Troy