I wouldn't add cider to the beer. An apple flavor, more specifically green apple, is a flaw found in poorly fermented beers. If you just want an apple ale, do that at serving time. Pour some cider in with your finished beer.
If you want to make hard cider, wait until the beer is done, then after racking the beer off the yeast, pour cup or so of that yeast into a freshly opened gallon jug of cider. You probably need to pour a glass of cider out first to make some space. Make sure your cider doesn't contain preservatives. Ideally some yeast nutrient would help, but if you pitch enough yeast, no new growth is really needed.
Cider fermentations don't need a big headspace, I think due to no head/foam forming proteins.
Not the textbook for great cider making, but it'll work.