Can an HDMI output be converted into something a USB input would accept?

External DAC's accept DSD only in two ways:

DSD over PCM (DoP) for S/PDIF and USB. Limited to 2x DSD

Direct DSD via USB 2.0. My Brooklyn plays at least quad DSD this way just fine. 

But to get here I have to buy DSD files I'm afraid. 

You may find more useful data here:

https://www.stereophile.com/category/music-round

When you go down the non-standard rabbit hole, don't be surprised if all you find at the bottom is rabbit turds.