Nature does have an "Open Access" option in their policy:

"Authors wishing to publish open access retrospectively should contact the individual journal editorial office, and will need to sign a new open access licence to publish form (LTP) and pay the journal's article processing charge."

http://www.nature.com/openresearch/about-open-access/policie...

However, they don't disclose how much money it costs to convert one's work to open access, and whether this option is available for all Nature journals, the main Nature journal, or just the specialized subject journals, such as Nature Machine Intelligence.