\setuppagenumbering[alternative=doublesided]

% You can use this instead of \setuppagenumbering[numberconversion=romannumerals]
% to set the conversion of the pagenumber outside of the sectionblock environment,
% the first argument takes the name of the conversion which is applied to the
% userpage counter with \setupcounter[userpage][numberconversionset=pagenumber].
%
% You can set a conversion for a certain sectionblock when you write the name
% of the block before the name of the conversion set separated by a “:”.
%
%\defineconversionset[frontpart:pagenumber][][romannumerals]

\startsectionblockenvironment[frontpart]

\setuppagenumbering[numberconversion=romannumerals]