said by rolande :

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They use static routing to the physical interface. The only way you could reuse user A's configuration is if you were physically connected to the same circuit. The ISP controls what traffic routes to where by the routes they add to their network. As a customer, you do not have control of these routes just by adding another user's configuration to your own network.

In a dynamic config scenario, the user authenticates either via PPP or PPPoE or something similar and all of the settings are passed to the client via a control protocol. The ISP's upstream router then dynamically inserts the new route into its routing tables and announces it to the rest of the ISP network using the local routing protocol. In that case, if you knew user A's username and password you could potentially steal their configuration and reuse it on another physical circuit and interface, since it is dynamically configured as a part of authentication.

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