Who added vowel sounds to the consonants of Hebrew?

There's no single person or event that "added" vowel sounds to the consonants of Hebrew. The development of vowel points in Hebrew was a gradual process that took place over centuries. The system we use today, a system of vowel points (niqqud), was largely formalized by Masoretes in the late Tiberian period (roughly 7th-10th centuries CE). They weren't inventing vowels, but rather developing a standardized system of notation to represent the vowels that were already being pronounced (though variations existed regionally). Before that, Hebrew was written solely with consonants, and the vowels were understood from context. So it's more accurate to say the Masoretes *systematized* the representation of vowels already present in the spoken language, rather than adding them.