Dear Huan Ting,
Thank you for your query.
I cannot explore in full right now due to conference travel.
I have escalated it to an ADN case 12276105 [IFCExportOptions.FamilyMappingFile is not working] for my colleagues to take a look.
Off-hand, I would ask:
Have you looked at the open source IFC implementation?
That would probably enable you to modify the way this mapping is handled to suit your needs with total freedom.
You say that clicking the button enables the functionality you need.
Have you tried clicking the button programmatically, or does that entail too much other complex user interaction?
If it is just a button click, you can easily simulate it using the Windows API:
http://thebuildingcoder.typepad.com/blog/2009/10/dismiss-dialogue-using-windows-api.html
I hope this helps.
Best regards,
Jeremy