Submitting sitemap for 404s

[user072319]

We are creating a Shopify app and we want to remove all 404 pages of our subscribed stores. When I researched I found that we can remove 404 pages by submitting a sitemap with all 404s(I don't want to do this manually say from GSC).

For a shopify store how can we achieve this?

1) As Shopify is creating the sitemap can we edit a store's sitemap?

2) What should we do? Create another sitemap and add it to existing sitemap.xml file?

3) Once we submit sitemap with all 404 pages, how long will it take to get removed from Google?