Live sand for saltwater systems is calcium carbonate.
Pool filter sand, builders sand, and play sand is any mix of quarts and silica sand. The silica part is an extreme no-no in saltwater systems. It will fuel diatom outbreaks until the end of time. It will drive you so nuts that you will either join the average saltwater hobbiest by leaving the hobby within six months, or you will trash it all, start over, and do it right.
So the answer is yes, you can mix them. However, just because you can, it doesn't mean that you should.