Well, at first i tought that all keyboard mapping in Fusion were global. Than's not true.
You can define shortcuts that are active when specific UI elements are in focus.
For example, if you want to define a shortcut that acts only when timeline has focus, you choose in the "Target" tree the "Timeline" node and define the shortcut there.
But if, like me, you want to be able to move to next/previous frame regardless where you focused (say you are refining a mask and need to keyframe it frame by frame), better if you define the shortcuts in the parent node, the "Views" node. This way the sortcut will be active regardless the View "type" has focus.
For the actions: i bound "Step backward"/"Step forward" to my J/K keys and i'm happy.
Of course it's intuitive once you read the tree header which says "Target". I didn't read it at first because i tought that, like other apps, the tree were the "Available actions" tree, not where to apply them
My fault.