Can BSA 68/73 bottom bracket be used in place of fat bike BSA 100 mm
The spacing is the only difference across the SRAM/TRUVATIV GXP line of BB's, so yes, you can use the 68/73mm left/right cup set on your frame. Just reuse the center seal/spacer from your existing BB, especially if you're riding a lot in the winter or rain.

Funny enough I've seen cheaper prices for the less widely used 100mm than the far more common 68/73 so it might make sense to just buy the right width unless you've got a good BB source!