Here is the email from Zoom with instructions for the Zoom H6 overdub using the on board mic/capsules. I didn’t have much luck with this solution working, going to try recording 2 stereo tracks at a time, one blank and one with the H6 on board mic/capsule along with the L/R recording, then go back and switch the file names and rebuild. It would seem that the Zoom H6 is looking for files with similar structure and time length making Step #3 the problem here. I’ll have to try it and see.
Sent another email to Zoom to see if the Zoom H5 has the same limitations. I was told, by Zoom, that the capsules are hardwired into the unit, preventing the overdub using them but, some online users are claiming that the Zoom H5 doesn’t have the same overdub limitations.
Customer Support response from Zoom:
You should be able to add .mp3 and .wav files to your H6. You will need to make a new recording that is set up the way you need it to be. This will depend on what you are importing.
For example if you want to add a stereo file to Tracks 1/2 you will need to arm them and create a temporary recording. Make sure you use the same format as the file you wish to add.
1. Choose your audio format in the H6 menu.
2. Arm the tracks that you wish to add your audio files to.
3. Make a temporary recording. It doesn’t have to be long.
4. Connect the H6 to your computer and select SD Card Reader in the menu.
5. Select The Folder and Project you are working on. You should see the temporary audio file that you created along with a data file. Remember the exact name of the audio file.
6. Delete the temporary audio file that you created.
7. Add the audio file that you want in your project.
8. Rename this file. Make sure you give it the same name as the audio file that you deleted.
9. Press menu to exit SD Card Reader Mode.
10. In the Project Menu select Rebuild and Yes to confirm.
11. Return the main screen and press play.
12. You should now be able to hear the file you added in your Project.