I don't have a 770, but you can do this with the pre-microb browser by editing ~/.opera/input.ini; look for the [Browser Widget] section, and change the text to the right of "Down" to be " = Page down" and to the right of "Up" to be " = Page up".

There's a way to do this on microb, but only by toggling the up and down to pageup and pagedown globally. (At least, the only way I know.)