Is this the Remembering the Kanji 1 list? When you delete a list, the progress of the words are still saved for if you re-add the list back later, and progress should catch up once the list is added back. If you want to add more words on to the end of the list, you could create a new section and add words to it.
If aren’t the creator of the list, you would need to create a duplicate copy (that you become the owner of) in order to add new words. You can do this by using the “Copy” link, at: https://skritter.com/vocablists/view/112160761
Also, here’s a blog post you might be interested in! https://blog.skritter.com/2017/03/shouldnt-study-single-kanji-japanese-not-high-frequency-word/