If you bump the alcohol too much...you may not have to backsweeten at all. But there aren't any issues with adding additional juice at this point. You may not end up with a reliable gravity reading unless you are proficient with math though
I have a batch that is aging now where I fermented my base apple cider (to 1.020) and then racked onto some peach with brown sugar. No clue what the abv is, but it loved having the additional sugars late in the party.