CUE files are plain text and can be edited or created by hand in any text editor, though if you don't have all of the audio offsets you'll obviously lose CDDA audio. The proper format is listed on the
Genesis Plus GX documentation page on libretro.com.
There also exists a program named "Sega Cue Maker" to generate a cue file from existing binary and audio tracks, provided that the filenames are numbered. This is kind of old now though and not optimized for the format preferred by RetroArch., but if you have ISO/WAV or ISO/MP3 but no CUE file you can make one with this and it will work in most emulators.
Probably the best method now is to just get a ReDump image and then convert it to either ISO/OGG or CHD (though I don't know if CHD is supported on Wii U).