Yes you can add flavor. I personally stay away from adding juices. Couple of things you could do if not yet bottled:
- Add Spices 2 Whole Cinnamon sticks, 1 Sliced Vanilla Bean, 1 Whole Clove and 1 oz Medium Toast Oak chips for a week to 10 days work great with Cider. Place them in a sanitized mesh bag add a few marbles to sink it.
- Add additional whole fruit 1 - 2 pounds for each gallon. Fruit with small seeds, strawberries, blackberries etc 7 - 10 days. Fruit with no seeds Blueberries, Peaches (Pitted) etc. 15 - 20 days. Freeze and cut into small enough pieces to get them in your carboy or again place in a mesh bag and sink them.
Rack when flavor is where you would like it.
If bottled consider adding a drop or so of fruit flavor extract to each glass when poured. You will need to experiment with this as I have no experience but imagine it would work just fine.