Hi katherineamt,
What you want to do will not work with RuneAudio. There are also lots of posts on internet documenting that this will not work, mostly relating to playback in cars.
However, I have a feeling that the basic concept should work, you could look here for details of something similar:
http://archimago.blogspot.com/2018/01/measurements-apples-lightning-to-35mm.html
If it works for an Apple DAC why not another USB DAC.
janui