Once you have a SuperKey ready for use, you can use it in half-keyed JOINs, as demonstrated in this code (contained in SuperKey3.ECL):

IMPORT $;

r1 := RECORD
  $.DeclareData.Layout_Person;
  $.DeclareData.Layout_Accounts;
END;

r1 Xform($.DeclareData.Person.FilePlus L, $.DeclareData.AcctSK R) := TRANSFORM
  SELF := L;
  SELF := R;
END;

J3 := JOIN($.DeclareData.Person.FilePlus(PersonID BETWEEN 1 AND 100),
           $.DeclareData.AcctSK,
           LEFT.PersonID=RIGHT.PersonID,
           Xform(LEFT,RIGHT));		
 
OUTPUT(J3,ALL);