If HGH levels in my body increase at 17-18 could I possibly get taller following a balanced diet & exercise?

Probably not. For you to increase in height your bones need to grow and once the growth plates close this is impossible. I am sorry to tell you that generally growth plates close 2-3 years after the start of menstruation so unless you did not start having periods until age 15 your growth plates have closed. Once that happens you can not grow any more no matter how high your hgh levels are.