Adding music to an iphone from multiple computers
It seems that iTunes Home Sharing would fit the bill for what you're trying to do. Here's the link for the basic requirements. I've done this before myself to consolidate my music library from several different machines. Since you own both machines, you should have no problem sharing your library from your PC to your Mac, and then just syncing from your Mac. However, both machines are required to be using the same local network.