\RequirePackage{template}

\DeclareTemplateType{xxx}{0}

\DeclareTemplate{xxx}{test}{0}
 {
  xkey=n \xkey,
  key =i {yyy} \value
 }
 {
   \DoParameterAssignments
   \show\xkey
   \show\value
   \value
 }

\DeclareTemplateType{yyy}{0}

\DeclareTemplate{yyy}{foo}{0}
 {
  keya =L \foo
 }
 {
   \DoParameterAssignments
   \show\foo
 }


\DeclareInstance{xxx}{bar}{test}
 {
  xkey = \UseTemplate{yyy}{foo}{ keya = 20pt },
  key = \UseTemplate{yyy}{foo}{ keya = 10pt }
 }

\ShowInstance{xxx}{bar}

\UseInstance{xxx}{bar}

\stop