You could use ffmpeg

ffmpeg -i test.wav -af "pan=stereo|c0=c0|c1=c0" out.wav

To install ffmpeg:

batch things are most flexible with a CLI.
it’s easy on Mac:

install homebrew:
http://brew.sh/

using homebrew, install ffmpeg:

$ brew install ffmpeg
use ffmpeg to batch convert:
$ cd ~/path/to/your/original/files/
$ mkdir stereo
$ for i in *.wav; do ffmpeg -i $i -af "pan=stereo|c0=c0|c1=c0" ./stereo/stereo-$(basename $i); done

If you'd rather not use brew, then you could probably use afconvert (built in OSX tool, uses CoreAudio frameworks):

$ cd ~/path/to/your/original/files/
$ mkdir stereo
$ for i in *.wav; do afconvert -d LEI16 -c 2 $i ./stereo/stereo-$(basename $i); done

-d LEI16 is wav format 16 bits (little endian, integer). Note - I'm guessing here. I don't have afconvert or a Mac. I'm not sure if "-c 2" will automatically replicate the mono channel to both channels or not. Or if the amplitude will be decreased (and if that's what you want) when the mono is converted.

For the ffmpeg example, I used the "pan= ..." part to make sure that L and R were copies of the original mono sample without making any amplitude changes to the sample.

The above was borrowed from a couple of sources:

https://www.elektronauts.com/t/batch-conversion-of-samples-os-x/16143/3
(note - above link is going between stereo to mono ... adapted for the reverse.)
https://sound.stackexchange.com/questions/42709/why-does-ffmpegs-conversion-from-mono-to-stereo-lower-the-volume
(above is reference to the ffmpeg command-line to keep the amplitude of the L and R channels identical to the original mono sample)
https://ss64.com/osx/afconvert.html
https://wiki.lazarus.freepascal.org/macOS_Sound_Utilities

Hi Kenneth,

Before spending anytime with alternate methods, you might want to learn about how to import samples into the MONTAGE/MODX.

The MONTAGE (and the MODX) can only import stereo .WAV files.

This is a false statement as you can definitely import mono samples into the MONTAGE and MODX. One reason we participate in the forum is stop misinformation, such as this, from traveling too far. There is a method this....

Where rumors begin: The Performance Recorder is capable of creating, loading and playing stereo .wav files. It creates a stereo 24-bit/44.1kHz .wav and is capable of loading and playing both 16- and 24-bit stereo .wav forms. That is the Performance Recorder.

The synthesis engine in both the MONTAGE and MODX are, for the most part, made up of mono samples, and you certainly load and create your own Waveforms from mono or stereo .wav data. The Performance Recorder creates and plays back stereo, only. The Synth engine, contrary to the misinformation, can deal with mono or stereo data!

If you give us an idea of what you are attempting to do, we can provide specific details on how you can import your mono samples. If you receive an “Illegal file!” warning message, this is because you have attempted to load your data to the wrong location within the architecture of the instrument.

For example, if you wanted to load a Mono sample to a Drum Kit Part or a Normal Part, you should be in that structure when you attempt to load the data (context sensitive).

A MONTAGE/MODX Waveform assembles a group of samples into a format that can be accessed by an AWM2 Element.
An Element can house as many as 256 mono or 128 stereo samples.
There are 8 Elements per AWM2 Normal Part and 73 Elements per AWM2 Drum Kit Part.

Within the Element’s Waveform you can define the mapping (where and how a sample will be triggered)... this is referred to as a KeyBank. Basically, a KeyBank defines the Note range the sample will be able to cover and the Velocity range that will cause it to trigger.

What you want to do is import your mono or stereo .wav directly to the Element.
If you are building a new musical instrument, you can start with the AWM Normal Init — or you can start with the Drum Init. You can also import directly to an existing Kit, (each Key, C0~C6, in a Kit is autonomous—each Key in a Kit can access one Waveform, but you can stack multiple samples vertically (Velocity will define *when* they sound).
Navigate to PART EDIT
If Normal AWM2 Part - find "New Waveform" on the "Osc/Tune" screen
If Drum Kit Part - find "New Waveform" on the Drum Key "Osc/Tune" screen

Rule: two samples (maximum) can share a Keybank ... that is to accommodate stereo. Translation: two mono samples, one left, the other right, or one stereo sample = a KeyBank. If you set more than 2 mono samples to the same KeyBank only the first two will sound.

What you want to do is load your mono samples by, first, entering the Part, selecting the target Element (there are 8 in a normal instrument, 73 in a drum kit), then tap the box labeled “New Waveform” — this action takes you directly to your USB stick. Select your .wav and proceed to loading the data.

Hope that saves you a lot of time and (unnecessary) effort.
Let us know.