They already clearly mentioned on their website any new information or additional documents will not be considered once you click on submit button and pay the fees. So double-check everything before you submit your application.
Hard to believe that you forgot to mention US refusals when the question clearly asks for it. These things are etched in your memory forever. The question is there to jog your memory. Even if you forget about it, it is hard to convince a visa officer to verify your true intention a second time. It is not like face to face interview. The decision is made on presented documents.