Logically an adaptor shouldn’t work because the bearings are completely different. In reality it probably does because the seals usually fail long before the bearings actually wear out in mountainbike applications so actually the perfect solution isn’t really required.
Shimano BB’s use angular contact bearings and the cranks Pre-load them slightly like a headset so they take both axial and radial loads. In engineering terms it’s the best solution, but it’s hard to manufacture so only shimano do it, everyone else just relies on 2 normal cartridge bearings and accepts that they’re not ideal for the axial loads. The latter (hope etc), can be converted because they’re using similar bearings to the GXP cranks, just in the wrong sizes.
SRAM BB’s have 2 different sized bearings, the drive side is designed to take radial loads only (rider weight and drivechain loads), the non-drive side bearing is clamped by the two parts of the crank so takes the riders weight plus any axial (sideways) loads.
Sell the shimano ones and buy GXP BB’s, the Gusset ones are recommended a lot and the bearings are replaceable and the outer seals appear to work well.
