I noted this is non trivial. Perhaps as the first cut, may be sufficient to just import Markdown based on the Headings as one node each at the related level. If the Heading has bullets/paragraph include them as multiline markdown text in the node. IMHO. ;-)

Also asked here: https://discuss.checkvist.com/t/export-import-via-markdown-format/446/2

Some implementation notes requested: https://discuss.checkvist.com/t/my-thoughts-after-over-a-year-on-checkvist/246/9