Thank you but SNote 2263653 - "Planning and Maintaining SAP Systems based on SAP NetWeaver 7.5 and higher" actually explains the problem. We do not have the choice but to use online Maintenance Planner (apps.support.sap.com/mopz) to generate the XML file, or push the required packages in the basket.
However we don't know how to "give" this .xml to SEA... Manual confirmation from MopZ of the files does not seem to work.