WoW-Mania

As the v3.3 game wasnt intended to have flying mounts there are numerous places where there are no textures on the hills and mountains. Is it possible to import textures from later game versions?

To be honest I highly doubt it, since many of those zones were "changed" permanently in Cataclysm. The main reason for the lack of texture in those areas (when you think about), is they are not part of any flight paths, the finished areas you see while flying over are related to existing flight paths in Wrath.