If you sync via Dropbox, you should therefore be able to use a Dropbox action from Drafts.
If you are using some other mechanism, as long as you can locate the folder on your Mac to write to, you can output to it.
There’s an example in this thread for prepending to a file outside of the Drafts sandbox, I might even have posted one for just writing to a new file at some point using the same technique.
But as far as I can tell, you should be covered for doing exactly what you want to do.