Certainly she is old enough to begin solids, if she is interested. High calorie solids such as avocado, whole wheat pasta tossed with olive oil, banana, etc., are a good way to allow baby to eat solids while ensuring the foods she eats are calorie and nutrient dense.
Expressing milk and freezing it in popsicle form is another fun way to increase breastmilk intake and allow baby to participate in meal time.
If baby eats pureed foods, which you don't necessarily have to do if a baby is a bit older when starting solids, you can add flax seed oil to the food to increase fat and calorie content.
I'm not sure of any additional ideas to encourage more breastfeeding that haven't been shared in the past: nursing in a dark room, nursing necklace to try to keep her interested at the breast, nursing when sleepy, using breast compressions while nursing, which can help baby stay interested in the breast longer and also have been shown in studies to increase the amount of fat in the milk.
Does she drink from a bottle or sippy cup? If you're very concerned about her weight (are you concerned or is doc concerned?), you can always supplement with expressed breast milk in a cup / bottle, if she's too busy to nurse well.
My own kids have both been well below any established curve by the time they were 9 mos old, so not being on the chart doesn't concern me personally. But I would evaluate her by the way she looks ... does she look healthy? Are her nails and hair growing well? Does she have adequate diaper output? Is she growing in length? Is she gaining weight at all - even if it is slow? Is she meeting developmental milestones?
If I felt comfort with the answers to the additional questions, I'd not worry about the weight and / or where it places her on the growth curve, but if she was somehow unhealthy, not making adequate diapers, not developing at a normal pace ... then I might feel more strongly about intervention. At 9 mos, I'd continue to focus on adequate time at the breast, but I'd also be offering high calorie healthy solid foods (although I had one with an eating disorder so this wasn't an option ... and I had to go to serious round the clock nursing to ensure adequate nutrition ... with her we also saw a nutritionist and our nutritionist was very confident that she was healthy despite being more than petite and eating nothing but breastmilk).