Can HBr add to a non-sp2 hybridized carbon?
There is a carbocation rearrangement. Basically, the H can produce two secondary carbocations. One of those secondary carbocations can do a 1,2-hydride shift, which produces a tertiary carbocation. This carbocation, being tertiary, will react much faster, resulting in being in the major product. So, yes, HBr can add to a non-sp2 carbon.