If the number of column is fixed, you can hard-code the means,
if the "etc." is variable, you probably need some Insert Into and Eval(Substitute()):
dt = Open( "$SAMPLE_DATA/Semiconductor Capability.jmp" );
offset = 5;
nc = N Col( dt );
length = 10;
groups = Floor((nc - offset) / length);
For( group = 1, group <= groups, group++,
myFormula = Expr( Mean() );
For( i = 1, i <= length, i++,
Insert Into( myFormula, NameExpr(As Column( offset + (group-1)*length +i ) ))
);
myName = "average" || Char( group );
Eval(Substitute(Expr(new Column(__myName__,Formula(__myFormula__))),Expr(__myFormula__),NameExpr(myFormula),Expr(__myName__),NameExpr(myName) ));
);