If you are talking about adding method code to a class you should use the Decorator pattern. Something like:
-----------------------------------------
CREATE CLASS decoratedThing AS custom
oThing = .NULL.

PROCEDURE Init
This.oThing = CREATEOBJECT("OriginalThing")
ENDPROC

PROCEDURE OriginalFunction
This.oThing.Function()
ENDPROC

PROCEDURE DecoratedFunction
** You could add code here.
ENDPROC

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Steven Black's web page will tell you more.
www.stevenblack.com