Hi Scot,
Welcome to Yamaha Synth!
What you want to accomplish is easy, once you understand exactly how the MOXF works as an audio device connected to a computer.

If you wish to use the MOXF as the audio interface for your computer, it can do so for both MOXF sounds and for anything connected to the MOXF's A/D Input jacks on the back panel. In order to output any of these sources you must place the sources in the MOXF's internal 'mixer'. This internal mixers addresses the sources via an entity called the "Part".

It is found in the Performance mode and the two Sequencer modes, Song and Pattern. By placing a Voice in a Part you are doing a similar thing to connecting an instrument to a channel of a mixer. Each of the Parts has the ability to have Effects inserted, can Send signal to Reverb and Chorus processors, etc. you can also route the Part to the available USB audio outputs, USB 1/2 or USB 3/4.

A single Voice in Voice mode can be Output to the main analog L/R Output.
But once in a Part you can additionally route it Out via USB to be recorded in the DAW software.

How to proceed:
Create a User Performance or Song/Pattern Mixing with the sound, or sounds, you want to record, in the Parts of the program.

Think of the Voice as the individual musician/instrument, think of the Part as a mixer, a means of connecting instruments to the recorder. You will just be using a single channel (slot) on the Mixer. Make a Song Mixing setup with the Full Concert Grand as Part 1.

You can have four instrument Parts in a Performance, plus the A/D input
You can have sixteen instrument Parts in a Song/Pattern Mixing, plus the A/D input

A step-by-step basic guide: MOXF Basic Audio Recording