Generally it is best to wait to introduce cow's milk until 12 months because of allergy issues. However, children that have been soley breastfed and had no other kinds of milk can generally be supplemented with cow's milk around this age as long as the majority of milk still comes from Mom. It is a bit controversial, so you can make the decision, but alot of the nursing experts say it is fine.
http://www.kellymom.com/nutrition/milk/milk-supplements.html
http://www.kellymom.com/newman/10bf_and_other_foods.html#...
I worked from the time my son was 10 weeks and pumped for him until he was 14 months, and then still nursed at night and naptimes until 29 months. Toward the end of the pumping, I just gradually added a little more and a little more cows milk each day to the pumped milk in his bottles and the switch was very easy. About a month after he finished the switch, we stopped with the bottle and he started drinking milk out of a cup, although he did prefer it to be warm. We had some minor constipation along the way but nothing major. You can always give some juice or a suppository if constipation does happen.
Good luck! And congrats on nursing 11 months! That is a great achievement! If you feel comfortable with it and your child seems to still want it, I encourage you to continue. There is nothing magical about the 12 months age that suddenly makes your milk no longer as good for her. It is still the best nutrition for her and will always have your antibodies in it to help keep her from getting sick. Also, when they do get sick, often, nursing is the only thing they will eat so I was glad that I kept it up.