Hi Adam,
Nice example, now I understand the cause of my problem.
I'll have toread again Wedening and Narrow Casts concepts, becauseI misunderstoodthe terms 
My requirement is add new functionality with out replace standard methods, because we dont want affect standard programs.
I did a Enhancement Class to add Z methods in CL_CRM_MKTPL_CPG_FUSG_COLL and can solve some points, It looks good.
Regards 