How can I enlarge a hole in a brick wall without drilling?
Technically you could use some device for precision grinding like Dremel with some diamond bit with an extender to make it long enough, but that's almost mission impossible to do. First, you risk damaging the existing cable. Second, doing such job in a hard material when the hole is long and narrow will take too much time and precision to be worth it.

You actually have two options. Option one is to drill another hole or maybe find some other way to lay the new cable. Option two is to disconnect the cable, remove it, drill a wider hole in place of the existing one, then install the old cable and the new one.