You could add a sweetener when you serve it. Simple syrup and agave nectar would be good choices because they are very sweet and water soluble, so they dissolve easily when you stir them in. You could use honey too if you prefer that flavor, but it doesn't dissolve well in cold liquid.
I don't see any good reason you need it in there before bottling, and doing so will make your life more difficult, requiring you to resort to (somewhat) exotic ingredients that won't ferment, potentially resulting in bottle bombs, and it still might not work.
I've been having the same problem in my fermented non-alcoholic ginger beer (very low alcohol soda) - it's not sweet enough. I'm planning to try three things with mine next - sweeten at serving as I said here; shorter fermentation (was doing 48 hr, going to do 24 next); and making it without fermentation (mixing seltzer and a concentrated concoction). I don't have a keg setup otherwise I'd force carbonate.