Hi,
I do not know how to do that properly (if someone has a solution, I'd also be interested), but here is something which seems to give a partial solution:
```
\usemodule[ipsum]
\starttext
\placecontent
\section{Section 1}
\dorecurse {6} \ipsum
\pagebreak
\section{Section 2}
\def\romanpage{\convertnumber{romannumerals}{\userpage}}
\setupuserpagenumber[
numberconversion=romannumerals,
viewerprefix=\romanpage -,
]
\dorecurse {6} \ipsum
\stoptext
```
In section 2, pages are shown with roman numerals in the document and as (roman numerals)-(arabic numerals) in the outline with Okular and Evince. I don't know how to get only the first part, though.
Best regards,
Florent