Adding credits after Construction Decision

Assuming this is the Construction Final and you haven't yet officially accepted it, yes - you can do that. The only way to add them however, is through an Appeal process. Just click the Appeal Review option and you can attempt/pursue more credits. You will be charged the extra per-credit appeal fee. Additionally, these types of appeals are handled just like other appeals (i.e. You get one submission shot with no ability to provide extra clarifications). As such, you'd want to make sure these new credits are documented really clearly so there aren't any questions as to whether the project met the requirements. You'd hate to have to go back and have to re-Appeal them again to provide clarifications.

If you have officially already accepted the rating, then you're stuck as the project gets locked down and the ruling is final.