There is no one way to proceed. You have many options available to you and you should explore them to find the different creative possibilities. How you ultimately choose to work should be tempered by what you need to accomplish.

You can continue to use the Apogee as your audio interface for the U87.
You can simply add the Montage as its own audio device.
You could build an Aggregate Audio Device that includes both the Apogee and the Montage as Input devices.
(The Duet is a 2-in/4-out audio device, the Montage is a 32-in/6-out audio device).
You could possibly setup to run the analog outputs of the Apogee to the A/D Input of Montage (using the +48v capability of the Apogee) thereby feeding audio to your DAW via the Montage. (This will depend on the capabilities of the Apogee).

A lot will depend on if you need to record both the microphone and the Montage simultaneously or not. In small home studios this can be an important factor.
Or do you wish to process your mic input with Montage effects, motion control, vocoder, etc.

You can, once you understand the routing possibilities, reconfigure your system as you require. Don't lock yourself into just one way of working. Experiment and discover the possibilities. If you have specific questions, please don't hesitate to ask.

You could also use a phantom power injector and plug the mic into Montage through the phantom power injector.

The best adapter would have XLR on the MIC input side and TRS on the output side with the option to use USB power, a battery, or an external ("wall wart") supply. Such an option may not exist.

Mostly you will find XLR-to-XLR injectors where you can get an XLR-to-TRS cable to handle the connection from the injector to Montage. Using a balanced connection will help keep the signal clean. Shorter cable runs help with this as well. Something like a Behringer Micropower PS400 or Rolls PB23 or Nady SMPS-1X or Sterling Audio PHP1 or Audio-Technica AT8801 or Ohuhu 1-Channel Phantom Power Supply or ... many other choices. You can spend nearly 200 or as little as around 19 on these.

I didn't spend a lot of time, but there's at least one XLR-to-TRS adapter that requires no adapter cable since its output is TRS (you can use a TRS to TRS cable). The downside is the power is only through a 9V battery and also that, as a "DIY" option, I'm not confident the designers have produced the cleanest solution in treating the audio path. This option is called the "Espresso Portable phantom power supply".