Download http://www.nexusmods.com/masseffect3/mods/76/? (just makes it easier to interpret all the info from files, not actually modding the game).

¤ You will want to have rebound previous/next weapon prior to doing this (the default mousewheel binds) if you haven't already, and make sure all your other keybindings are set up properly as well (you can do this in-game).

¤ Open Editor, go to BioInput->SFXGame->SFXGameModeCinematic and add a binding for either or both of MouseScrollDown or MouseScrollUp (this allows cutscene skipping to be rebound).

¤ Go to BioInput->SFXGame->SFXModeConversation and copy the row with "SpaceBar" to a new row, and then replace "SpaceBar" in the new row with "MouseScrollDown" or "MouseScrollUp

¤ Save the changes.