When I select multiple bar lines and add a break, only the first bar line receives a break. Would like all bar lines to receive a break simultaneously.
That’s by design. When you add a break, Dorico recalculates the casting off of the other systems (I’m not saying your suggestion is a bad one).
Maybe what you want is fixed casting off? If so, you can set that in Layout Options.
Note that (within Write mode) you can select an existing Break and Alt+click it to other positions. This is likely just as quick as the method you’re requesting.