When adding muscle mass anywhere in the body you get added support which translates into added and improved posture.
If your shoulders slouch forward too much then your rear deltoids and upper back might be lacking in its role to bring you proper posture in your shoulders.
This sounds like simple muscle imbalance. You should train all your muscles regardless of this condition.
I'm not a specialist though and you should see one if you haven't already.
Oh - sitting on a chair in front of a computer is bad, I should know - I do it all day long That's why I goto the gym 4-6 times per week.