Have you tried an online service such as
https://www.freeconvert.com/wma-to-mp3? Easy alternative to a full blown media conversion suite, if it works with your files.
Once the files are in MP3, you just copy them to the USB drive, folder structure doesn't matter (will be scanned and detected by the infotainment software), but the drive must be formatted to FAT32.
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2018 Camaro 2SS — G7E MX0 NPP F55 IO6
735 rwhp | 665 rwtq
Magnuson TVS 2300 80mm pulley | Kooks 1 7/8" LT headers | JRE smooth idle Terminator cam | LT4 FS & injectors | TSP forged pistons & rods
JMS PowerMAX | DSX flex fuel kit | Roto-Fab CAI | Soler 95mm LT5 TB | 1LE wheels | 1LE brakes | BMR rear cradle lockout | JRE custom tune
1100 - 1/30/18 | 2000 - 1/31/18
3000 - 2/06/18 TPW 2/26/18
3400 - 2/19/18 | 3800 - 2/26/18
4300 - 2/27/18 | 4B00 - 3/01/18
4200 - 3/05/18 | 4800 - 3/14/18
5000 - 3/16/18 | 6000 - 3/19/18