FWIW it does have to be a package if it contains any attachments, but if there are none, you can look inside the package and extract the contexts.xml file (compressed or otherwise, see Inspectors > Document > Advanced), change its extension to .oo3 and use it outside the package as a flat file.

It will continue to be flat after further editing and saving, unless you trigger a metamorphosis into a package by adding an attachment.