Creating a SuperKey is exactly the same process as creating a SuperFile. The following code (contained in SuperKey2.ECL) creates a SuperKey and adds the first two payload keys to it:

IMPORT $;
IMPORT Std;

SEQUENTIAL(
   IF(~Std.File.SuperFileExists($.DeclareData.AcctSKname),
      Std.File.CreateSuperFile($.DeclareData.AcctSKname)),
   Std.File.StartSuperFileTransaction(),
   Std.File.ClearSuperFile($.DeclareData.AcctSKname),
   Std.File.AddSuperFile($.DeclareData.AcctSKname,$.DeclareData.SubIDX1),
   Std.File.AddSuperFile($.DeclareData.AcctSKname,$.DeclareData.SubIDX2),
   Std.File.FinishSuperFileTransaction());