Not exactly what i was looking for as the defaultmapping will not work with en event that is already of a specific class other than "/Unknown", however I've managed to find the solution to my problem: how to add transform to a class rather than class mapping:
"A class transform is configured starting from the Events lefthand menu, by drilling down to the class in question (not the class mapping) and then use the top dropdown table menu to bring up the More > Transform menu."
Now I can edit eventClassKey, message, summary and anything else I need.