First and foremost, you don't know for certain that it's ADD/ADHD. You need to find out by getting an evaluation by a Child Psychologist, Pediatric Neurologist, or Developmental-Behavioral Pediatrician any of which need to include in their specialties ADD and ADHD.
With neurological disorders such as ADD, ADHD, Autism, and more, diet CAN BE HELPFUL because if there are foods that your child is sensitive or intolerant to then those foods act as toxins and they can make the symptoms and signs of the whatever neurological problems someone is having worse.
So if you remove things like HFCS, artificial food colorings, refined sugar, and artificial flavorings you may very well see improvement. If you see a complete "recovery" then that indicates there wasn't a neurological disorder but an allergy/sensitivity. If symptoms decrease in severity/strength but still exist then you know the diagnosis is accurate.
This is the way these artificial "food stuff" work: a person that ingests them who is sensitive, intolerant, or allergic will have difficulty digesting them. Their liver and kidneys don't purify the blood and the toxins move through the entire body including the brain until the body can expel them. What happens during that time is that the person can have emotional upset, crying jags, aggression, rages, hyperactivity, difficulty concentrating, difficulty sleeping, nausea, racing thoughts, lack of focus, poor management and organization, overreactions... essentially like a mean drunk coming off of a three day binge. Because these toxins are exactly like alcohol and the response the body has when there are toxic levels ie. tipsiness, drunkenness.
You can do an elimination diet at any time and it won't harm your daughter. Even if your child is 100% typical it's good to get off of those things anyway. Clean eating is really important for the brain and physical development.
Behavior modification... well... you really need to know what you're dealing with and have a high degree of certainty. And initial diagnosis by a pediatrician and a pain management doctor (really?) aren't ones I would trust because they're definitely not qualified. Just like although a gastroenterologist initially diagnosed my Fibromyalgia, he wasn't qualified to treat me or make a formal diagnosis. He got me on the right track. It sounds like that's all the pediatrician did.