\setupinteraction
  [state=start]

\defineconversionset[frontpart:pagenumber][][romannumerals]
\defineconversionset[bodypart:pagenumber] [][numbers]
\defineconversionset[backpart:pagenumber] [][characters]

\setupuserpagenumber
  [way=byblock]

\setupuserpagenumber
  [numberconversionset=pagenumber]

\starttext

\startfrontmatter
    \chapter{Bla} \dorecurse{20}{\input zapf \par}
\stopfrontmatter

\startbodymatter
    \chapter{Bla} \dorecurse{20}{\input tufte\par}
\stopbodymatter

\startbackmatter
    \chapter{Bla} \dorecurse{20}{\input tufte\par}
\stopbackmatter

\stoptext
