I'm guessing your pool is gunite/concrete, since most in this area seem to be. The process roughly would be to fill the deep end with concrete, but that's easier said than done. You would need to jack hammer into the existing pool shell to get down to the rebar, and tie in new rebar for the new concrete. Simply adding concrete inside the existing pool shell would certainly crack eventually, the whole shell needs to be one solid reinforced structure. You would also have to jack hammer out the current drain, and plumb in a new drain at the top of the new floor. It would probably require two drains in the new floor, in order to meet updated building codes, but that could simply be a tee fitting off the existing drain line. You would then need to replaster the entire pool for a seamless finish. Cost-wise I'm not sure how much, but you would be looking at many thousands of dollars, maybe even $10k or more.