Is it okay to add some orange juice to formula for my ten months daughter? To add flavor

Not yet. Please discuss all dietary issues with your child's doctor for continuity of care, but at this point in her young life, the acidic nature of orange juice may be a little too strong for her developing intestinal tract. Best wishes.
No. Orange juice, similar to lemon juice, which is acidic don't generally go with milk, which is alkaline. It will most likely curd, if you let it stand. Babies less than a year should in general avoid citric juices as these may invoke allergic phenomenon in some cases.