Currently not possible to export content from a custom register in the same format as importing. All registers should allow the content to be exported in a portable format to allow customers to work with the data and upload it as necessary. Further, this can make it much easier to get vendors to review and update content without having to provide them access to the entire tenancy.
Importing content is supported; however the "Download Template" only provides a generic example. These should be generated at least from the specific register in question.
Essentially, it should be possible to Export Register > Make Changes > Import Register.