OK, I'm not exactly sure what I did, but my student was able to edit the example sketch from MySensors V2 "GatewayESP8266MQTTClient" and add in code to send data from the Adafruit BME680 over MQTT. Once I added that code to my NodeMCU and added the # to the subscribe list in my gateway, it all just seemed to automatically add.
I should acknowledge that I had a misunderstanding about what physical components were nodes, sources, and fields. For some reason I thought that my NodeMCU was a gateway (this is true), it was also a node (this was not true), and that the BME680 was the source with several fields on it (TEMP, PRESS, HUMM). I believe I see now that the Node is actually the BME680 board and it has several separate sources, the physical sensors for each of the conditions it monitors and those numeric values, once acquired, become fields that the MC server can publish.
I am going to play some more with this since I would like to add all sorts of other nodes with a wide range of sources.