Adding a two-way circuit to a three-way circuit
If both switches are of the type 2 configuration in your diagrams, then no. You need unswitched power from somewhere to either the new light or new switch. For example, if either of the switches in the 3 gang box were of the type 1 configuration, then you have unswitched power and can add another switch.

To do what you want you would need to run another conductor from where there is unswitched power available.