You can use iTunes file sharing to put the files into RSSRadio.
Connect your device with the cable, then in the apps section there is a “File Sharing” part where you can copy files.
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.
Using the sharing extension
See how to enable the sharing extension