How can I bulk import mp3's to RSSRadio?
You can use iTunes file sharing to put the files into RSSRadio.
You should place your files in the "files_to_import" subfolder for RSSRadio to correctly recognise them. Recent versions of iTunes may not be able to put files in the folder, in this case move the folder out to your desktop, fill it up then put it back.
You can load an mp3 onto something like the Dropbox or iCloud drive app and Open In... that file into RSSRadio.