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it should work all the backpacks are pretty much compatible nowadays

The Build Booster Mk II is an attachment that fits it onto existing backpacks on HGUC kits. You can use it on the Crossbone Gundam backpack as it has a single port where the crossbones would usually be attached. Unless the GM Sniper K9 has the original backpack on the runners you’ll have to find some way of attaching it with the K9 Dog Pack (there’s a port where the gun is attached in dog mode) that doesn’t look weird.

I pretty sure all Build Fighter Kits Backpacks are transferable that was the whole point of the line you can mix and match