To edit sketches to work for backpack and other accessories,
#define LED_TYPE WS2812B // optional
#define NUM_LEDS 64 // 40 leds back + (12x2) on the front sides
You can also use something like below to force the backpack to be a master, or FFFFFF for a slave
#define nodeID "000000"
Make sure the nodeID is unique for each backpack/accessory. Just increment/decrement it in hex.
You might want to make your single long strip into smaller, sub-strips. I'd recommend looking at this gist to get an idea of how you can modify the LED strip declaration to get started with that. Then you'd need to change all places where the leds[] is updated, to include an additional update for each new LED array you add.
Rory's Bike Flag: 22 LEDs long