Is it possible to make your character wear armor using nova skin?
erhm uh, what u really mean dude? explain pls <3
Can you, using this site, put armor on your minecraft avatar
only if the dude who made the whole wallpaper added more frames with specific armor so it allows u to add it. Else no, by the site’s way, use photoshop or something sorry 
are you talking about the editor or the wallpaper creator,
Editor:
usally people post armour on the wear tag on the left hand side http://i.imgur.com/SHHl6iG.jpg
Wallpaper:
Currently not possible unless the skin itself has it