The problem is not the import into drafts but drafts interpretation of the content of the file. i guess a different extensions will not fix that internal interpretation.
I guess I don’t follow what are you trying to do? I thought you were saying you wanted to edit the JSON in Drafts.
If you import a .draftsExport file you should get the import dialog and be offered the option to import the individual drafts.
The tools to write your own importer action are all in place now, with R22, the date properties of drafts can be set in scripts, so if you want to write a tool to do imports of other formats without the intermediary JSON files, that might be a better option.