You can do that easily with uTool .
Please download the uTool 2.0.34, in the settings select uTrack24. On the “home” tab start to create a new multitrack song from your mono wav files. Save the song then on the export tab select your USB HDD and export the song.
The uTool can be used for import recording from your USB drive and export songs (stereo or multitrack) to your USB drive as well.