Line in to phone adaptors are not that expense, so I personally dont see any reason to mess with overloading the original specs. Just go buy an adaptor if you dont have a line in. I'm using one to hook up my computer to the GF 9595, and it sounds fine.
Kind of a weird thing, but on one of my panasonic boomboxes, I prefer running a line in to a phono converter to the phono-in, even though the boombox has a line in. It just sounds better for some reason.