Yes, but this tag is not recognized by anyone and is kind of a trolltag (see https://wiki.openstreetmap.org/wiki/Trolltag). Data consumers are going to look at the highway=* value and think "okay, so this is a freeway at the topmost importance level". At that point, if they do not pay attention to highway:motorway=*, they will still treat the road like it is in the topmost importance level, even if it is not.