I don’t think opus can be stored in an mp3 container without reencoding. Opus can be stored in mkv, ogg (or opus, an alias of the ogg file format, unless that changed), mp4 (I think this may be experimental?), and webm, off the top of my head. Try using ffmpeg to copy to various formats: ffmpeg -i in.opus -c copy out.ext, replacing ext with whatever file format you need. -c copy will always be lossless.
You can also try reencoding as flac or another lossless audio codec, but this may balloon your file size.