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