You should check with the guys at SLIM Devices to confirm but I see no reason you cannot have five SB nodes on the network. The SB software has provisions for naming each node independently, slaving one to another etc. Their customer service is tops - no trouble getting a real human, at least before Logitech took them over.
As far as adding the two locations - you cannot connect the additional two locations through your main system... for that kind of approach you would have to use the zone capabilities built into some amps and receivers (but hardly all). At that point you would be running speaker cables - and would no longer be able to control your source (i.e access your hard drive) remotely.
Instead, simply hang the additional SBs on your Cat 5 network, or tie them in via WiFi using a wireless SB. This will maintain your ability to select program (i.e acess the hard drive) from each location independently.
I have never tried to use five SBs at once so I don't know if your hard drive will handle simultaneous playback of five different files... but it might, especially if its SATA and you're not doing anything else on the network or with the computer. You might also want to consider putting in a NAS so that the music network can operate independently of your computer. And of course you will need a pretty good size router.
Be very interested in how this turns out.