I’m not sure what you mean when you say that there is a foundation already built for your pool — pools are normally built in undisturbed native soil — but it is entirely possible for a gunite/shotcrete pool to be built completely above ground. However, such an installation would require special engineering. You would need to consult a structural engineer, preferably one specializing in pools, to obtain such an engineering plan. It might be wise, since you are building on a slope, to consult a soils engineer as well. It would be a bummer if you built the pool and it slid down the hill or something. You should know, too, that jobs like this — with soils reports and special engineering for unusual conditions — can turn out to be very expensive.