Fairly new to cider making. I've made a gallon of cider from cooking apples, it's stopped bubbling and is still in the demijohn. The juice tasted great, but the cider is not amazing. I'm just wondering if I can blend some of the juice in when I bottle it so it's a bit more sweet and appley. How would I do this without causing exploding bottles, but still get a decent carbonation?
The bad news: it's not possible to get a carbonated sweet cider without kegging or forced carbonation. The yeast is still active, so if you add juice at this point, the sugars in the added juice will just ferment and you're back to square one.
If you're okay with a non-carbonated (still) cider, there are methods of halting the yeast (campden tablets, pasteurization) and then you can back-sweeten with juice to taste. If you can force carbonate, you're fine!
If you're okay with dry & carbonated, you can add the typical amount of sugars when you bottle, to bottle-condition & carbonate. You might be able to get some more appley flavor if you prime using apple juice concentrate instead of sugar - note: I haven't tried this, but it should work.
The good news: a disappointing cider will often improve with aging. I find my ciders often taste fairly harsh early on, but they mellow out after a few months.
Edit: Philippe makes an excellent point below - you can certainly add a non-fermentable sweetener!