My interpretation of "folk process" is the unconscious or largely unintentional changes a singer may make to words or tune as a result of misremembering or misunderstanding the original. Where someone sits down and consciously reworks a song, whether it is pulling together a number of different traditional sources to make it more complete, or by adding bits of their own, that is something different. Whether or not it is plagiarism depends on whether or not they acknowledge their sources or claim it as their own.