Folks have tried different things. I'm not a fan of splenda either, in any form. Too sweet for my taste. I haven't use any non-fermentable sweeteners myself, but if you do a search for it there have been a few long discussions where people have brought up a bunch of different options. I just can't remember them off the top of my head.
Whatever you choose, I would add it at bottling. Right in the bottling bucket along with your priming sugar and then rack onto it and give it a gentle stir.