MODIFICATION: This is the fun part...
Having read a few people's attempts to install an H100 in/onto a HAF912, the only way is through modding. If you have rotory tool (dremel) and a few cut-off wheels, you can cut a hole large enough to fit the waterblock/pump through (be very careful putting the waterblock through so you don't scratch the surface) and mount the rad on top of the case - the hole should be lined up with how the rad fits so the waterlines will fit straight through once mounted. Some grommetting can be fitted around the hole to prevent chafing/cutting the water tubing. You would have to check that the water lines will be long enough to reach the CPU with the rad mounted higher.
At least that's the approach I would attempt, hope it helps
I can fit the waterblock in the case, but how would I be able to fit the radiator? The thing is that I only have about a inch from the top to the motherboard and only about half a inch between my drives if I use the front.