Can I hook my modem up to a hub and get online with all my networked Comcast PC's?

Everything can be hooked to a hub.

But alloting only one IP address by Comcast's standard HSI package allows you to use only one computer at a time.

However you can overcome this problem by purchasing more than one IP address with your account and then configuring each computer with a unique IP address by which the services can be accessed through a hub.

If you want to hook up your whole LAN, then looking into a router or some NAT software is not a bad idea.