The docbook2mdoc utility is a converter from DocBook V4.x and v5.x XML into
mdoc. Unlike most DocBook utilities, it's a standalone ISC-licensed ISO C
utility that should compile on any modern UNIX system. The only requirement is
libexpat (for parsing XML), which is installed by default on most systems.

WWW: http://mdocml.bsd.lv/docbook2mdoc/