it really depends on your bubs if formula will make her sleep longer at night. Some bubs it does help others not. She maybe going through a growth spurt at the moment and need extra milk.
If you do want to introduce formula, I would give it to her in her 6pm bottle, and then maybe a dreamfeed at about 9pm or 10pm.
Hope this helps
Yvonne, WA, Beautiful Baby Boy 29/11/2005