Hello Jane,
As a special ed. teacher who has done years of work with children who were born addicted to drugs, I can tell you that the possibility of that child struggling with a lifetime of issues is high. The studies I've read show children who were born drug addicted have low impulse control, the same symptom that shows up in adults doing drugs. Their brains are slow to regulate their behavior and thus appear ADD because they act on their every impulse. This is often compounded by cognitive impairment or delay which also brings with it a level of frustration to the child.
I have a close friend who adopted a boy 17 years ago who was addicted to opiates and the whole family is affected by his impulsive, explosive, and often violent behavior. I have a huge amount of respect for your willingness to consider taking this life long responsibility on, however, be prepared to deal with many potential issues, social drama, and restrictions on your ability to function as a normal family.
Good luck,
Wendy