Hi Laurent,
Thanks for answer.
Yes, I know using metadata I cannot change any data, I've only used metadata to change de visualization.
I've tried to modify the webdynpro adding a post-exit in method WDDOMODIFYVIEW and I can change the data, but only works if I push enter (or check or do something) before push save or release. If I add the data and then push save without push enter, then doesn't work because the method WDDOMODIVYVIEW is triggered after the method to save the data... 
I think with the BADI you said the same will occur.