As far as I read in amazon metadata downloader, I think it just update a Metadata object and then put it on a result queue (I still didn't figure who is responsible for getting it back). Then this Metadata is saved to database for the corresponding book which started the downloader in the first place.
This is why I was trying to add my values to this Metadata object to be saved along.
Code:
mi.set_all_user_metadata(um)
Thanks