Portable Recorder for Vocals

Hi Folks,

I haven't recorded in a while, but am ready to jump back in again. My process involved laying down tracks in Sonar, sans vocals, exporting to a WAV file and recording dry vox in my walk-in closet on my Zoom H6. I then process the vox back in Sonar. Alas, I have forgotten the procedure on how to record vocals on this unit, over prerecorded audio!
Per page 25 of the Zoom H6 manual, I have tried the following with no luck:
1) Formatting the SD Card on the Zoom. Then plugging the same card into my computer and placing the pre-recorded file (i.e. song, sans vox) in FOLDER01.
2) Naming the pre-recorded music track to "ZOOM0001_LR.WAV" (44k/16-bit stereo) and placing it in FOLDER01. Zoom cannot play it or see it.
3) Mixing the above file to mono (in a DAW) and naming it "ZOOM0001_Tr1.WAV" (44k/16-bit mono) and placing it in FOLDER01. Still, Zoom cannot play it or see it.
4) Since "Overdub" will not even be selectable if there is no track in a folder, as a last result, I tried placing a previous project file, naming it "ZOOM0001.hrpj" in the same folder. No dice.
I have scoured YouTube, looking for tutorials and it seems this function is well-overlooked. (Non-existent?) I am beginning to think that maybe I did this on my old Zoom H4n and is not possible on the H6. (Though why they would remove this function in a later generation is dumbfounding...)
Assuming that it's NOT possible on the Zoom H6, can anyone recommend a competing product that does support this function? I am currently considering the Tascam DR60 Mk II. I just have no idea if it has the capability I am looking for. Thanks in advance!
Ed

Thanks for your response, Cactus. I did get a response from Zoom:
"Its possible this was done on the H4n, but this is not possible with the H6. You would have to re-record the file on to the H6. Transferring in the manner described does not work due to metadata attachments on the H6."
Ironically, I am clearing the egg from my face as I sold my H4n to upgrade to the H6, assuming it had all the bells and whistles (and more). Now I need to downgrade.
To answer your question about my recording method,the room where my workstation/DAW is located has terrible acoustics (aka my office). So the best way to get dry vocals is to sing along to the existing track. No drift at all-it's all digital.
Related, a rep at B&H told me that the Tascam DR60 does NOT have this capability either. Luckily, the original H4n models can be got on eBay for about $125.
 

 No drift at all-it's all digita
 
There can always be drift if the recording is not synced to the same clock. Over a 3 min song it might not be more than a few samples so all is OK, but you do need to be aware that it can happen. And all clock sources are not created equal.