Hi Sai,
first create one zclass with the super class name is ur "CL_XXX_CN02.
drill down your ZL_XXX_CN01. class there is a method IF_BSP_WD_TREE_NODE~GET_CHILDREN copy the standard code and redefine the method past the standard code into your redefine method. if you observe in that method there a standard class name "CL_XXX_CN02" replace that class name to your zclass name..
ex: first class cn02 under there is method get_childern.
then create zclass with the super name here cl_xx_cn03.
go to get_childern.
replace that class name to your zclass name.
that will allow to change the getter nd setter methods.
Try this will work.
Regards,
Srinivas.