Learn French: Il a ajouté de la noisette.

De la is the partitive article used for uncountable quantities, like an ingredient or flavor. Using de la noisette means he added hazelnut flavor, paste, or powder. If he added whole, countable nuts, it would be des noisettes (some hazelnuts).

How would I say "He added a few of them" (meaning whole hazelnuts) using this lesson's grammar?

You would use the feminine plural pronoun quelques-unes (a few) because noisette is feminine. You also need the pronoun en (of them) to connect the verb to the quantity. The sentence becomes: Il en a ajouté quelques-unes.

If I say Il en a ajouté quelques-unes, does the past participle ajouté need an extra -es to agree with the feminine plural nuts?

No, it stays ajouté. Even though quelques-unes refers to feminine plural nouns, when the direct object involves the pronoun en (of them), the past participle in the passé composé does not make an agreement.