At the moment your only option is to write your own typescript for the
fonts.
\definefontalternative[sb]
\definefontalternative[si]
\starttypescript [sans] [source-sans]
\definebodyfont [default] [ss]
[tf=Sans sa 1,
it=SansItalic sa 1,
sb=SansMedium sa 1,
si=SansMediumItalic sa 1,
bf=SansBold sa 1,
bi=SansBoldItalic sa 1]
\stoptypescript
\starttypescript [sans] [source-sans]
\definefontsynonym [Sans]
[file:SourceSansPro-Regular.otf]
\definefontsynonym [SansItalic] [file:SourceSansPro-It.otf]
\definefontsynonym [SansMedium]
[file:SourceSansPro-Semibold.otf]
\definefontsynonym [SansMediumItalic]
[file:SourceSansPro-SemiboldIt.otf]
\definefontsynonym [SansBold] [file:SourceSansPro-Bold.otf]
\definefontsynonym [SansBoldItalic]
[file:SourceSansPro-BoldIt.otf]
\stoptypescript
\definetypeface [source] [ss] [sans] [source-sans] [default]
[features=default]
\definetypeface [source] [mm] [math] [xits] [default]
\setupbodyfont[source]
\starttext
Upright {\it Italic} {\sb SemiBold} {\si SemiBoldItalic} {\bf Bold} {\bi
BoldItalic}
\stoptext
Wolfgang