I think this is the best way to say it:
'Nine features were added'. 'Additional' is redundant, since 'added' is already there.
'Adding additional weight'....does that make sense? Of course not! 'Adding weight' is sufficient.
If 'additional' is used, the word 'an' is required because it is a single quantity. It must be 'an additional nine' in a sentence such as: 'an additional nine soldiers were asked to stay behind.'