Walls – this protrusion of bricks in the wall

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.

This Old House's website advises getting a sample of the current plaster and mortar to s lab so you can get them accurately matched, if I remember correctly. They seemed to be saying that it was important that the brick and mortar be matched to each other, tho I'm not sure why..