Can hashes be used to generate larger random numbers?
No, actually your 32-bytes would only be a representation of the 8-bytes random (or pseudorandom) number. In this case if I want to attack you system (eg. by brute force) I need only to perform 2^8 attempts instead of 2^32 as someone could expect. This occurs because the 32 hash is only a deterministic representation of a very small 2^8 number.