Caetano,
This is the problem - calling MD_STOCK_REQUIREMENTS_LIST_API inside the BADI. If I run MD04 with only the elements of the principal material but determined by calling the FM MD_STOCK_REQUIREMENTS_LIST_API from the BADI (no element is added) I receive the errors.
So I should determine the MRP elements for alternative materials ...."manually"...
...
I should ...kind of replicate the logic of MD_STOCK_REQUIREMENTS_LIST_API?? 
Regards,
Florina