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

Thanks for the responses, folks -- I'm afraid this ship won't float.  @whart you may be right, but the Cambridge tech support people said it was a "DSD" signal, so I assumed they meant unencrypted DSD -- you know what happens when you assume.  It sounds like there is no simple conversion, so I'll just continue to listen to my SACD's the old fashioned way (which sounded good, it's just a bit inconvenient).  Happy listening to you all!

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.