If the system is smart no you can't but most systems like this are stupid and assume a IP address only comes from a single machine.
All you do is put a normal router in place. Since the addresses are natted the first machine logs in and the service opens the IP address of the router not the actual machine. Then other machines can use that same connection since they are natted to the same address.
This of course only works for a period of time since most system like this require you to relogin from time to time.
Other methods use some form of tunnel which you cannot get past as easy. PPPoE is supported by most routers but other things are not so simple.