Well, almost anything is possible...if you're will/able to pay for it. You have some potential problems in doing what you want to do. The most obvious of these is the drain. A drain must slope 1/4" per foot or 1" per 4 feet, and I doubt if there is space to run the drain under the floor. If the drain would be over the garage, perhaps you could have it exposed under the ceiling, but a toilet drain must be at least a 3" pipe and can't just tied into a drain of smaller diameter. Me thinks you'd better contact a plumbing contractor anyway, because this is not a job for someone with no experience or knowledge, there are too just many pitfalls.