Drafts has an export action step which opens the file browser and can save to any of the destinations you have configured in the Files app. You do have to manually select the destination each time, and it is not possible to append to those files, only save new ones.
Is there a specific service you are trying to integrate with? Drafts has actions which can append to files via direct integration with iCloud Drive, Dropbox, Box, Google Drive, and OneDrive…or a WebDAV action that can work with files directly on any WebDAV enabled host.