Well the issue here is that Mojang changed the file type that map files are saved as. Orignally, they were saved in a format called "Region", and now they use a format called "Anvil". In addition to doubling the heightmap limit, they changed a lot of the algorithms used to generate worlds. Unless you can find a mod that inserts the new blocks and items into your 1.1 world, I don't believe there is any workaround.