Yes, you can add sand to an existing tank, just do it slowly. Don' use "sand" sand, use "aragonite" sand. The major brand is Caribsea, but there are others. It's available in various sizes (from oolitic, e.g. very fine, to 1-4mm mixed). The 1-4mm mixed has some of the looks of crushed coral (which I like); the 1-2mm size (Reef Floor grade, I think) is also fine. This is "dead" sand, as opposed to "live."
I would personally remove existing crushed coral substrate, because the particles are largish, and trap leftover food, making it difficult for bristleworms and other worms to live in the sand. The smaller grades are small enough to establish a pretty lively community of baby stars, bristleworms, peanut worms, cerith snails, pods, etc. (should you be fortunate enough to have them).
As ufans pointed out, "SLOWLY" is the watchword. When you add sand (by that, I mean aragonite) to an existing tank, you will generally create a dust storm in your tank. Pouring thru a PVC funnel, or other similar contraption, minimizes the dust storm.
Live sand in a bag, as far as I'm concerned, is $ in a hole. The only benefit it's got is bacterial cultures. I'm not (any longer) a biologist, but how can worms, snails, minicrustaceans, etc. possibly remain alive in a sealed polybag sitting on a store shelf with no expiration date??? If you want live sand, much better to borrow (beg, buy, steal) a couple of cups from a friendly neighborhood reefer (or nice LFS) to seed dry aragonite that you rinse. Or, buy real live sand, collected from the sea floor, by one of the aquaculture companies (e.g. Tampa Bay Saltwater). Collected live sand shipped from the Ocean is the best live sand. It's not pristine white, but then, your substrate won't be pristine white after a month

. But collected live sand is full of mini-life (mini stars, mini chitons, mini other mollusks, mini worms, mini crustaceans) that do a great job as detritivores.
How much do you need? Do either a search for DSB, or make a post asking about DSB's

Seriously, IMO, either go shallow (about 1/2") for looks, or go way deep (4-6") for nitrate reduction, but don't be Mr. In Between.
I'm sorry, I don't recall the cubic inches to lbs of substrate conversion.
HTH,
Hy