If you mean in addition to the music and movies you already have on your iphone, then no. iPhone prevents you from syncing with more than one itunes library at a time. It's kind of a bummer because you used to/can do it on any iPod and they dont hassle you at all. I hope this helps and isnt too disappointing
Yeah, I think if you connect your iphone to another computer and choose the option to manually manage your music, you can just drag and drop stuff from any computer. I haven't tested this out yet, but I don't see why it wouldn't work.
Now syncing it would be a different story. If you sync it, you can kiss your current playlists and songs farewell.