How to use volume keys for scrolling? (everywhere)
To my knowledge, there is no way to do this system-wide without heavily modifying the ROM, or at least system files. Especially if ButtonRemapper doesnt show your volume keys as programmable buttons, then it won't be easy. Some apps will, however, let  you customise their behaviour. For example, I know browsers such as Dolphin and Miren as well as Google Reader can change the volume button's behaviour.