How can I add MQA decoding with my current setup?

The way MQA is set up, if you want the fully unfolded MQA data, it has to be performed by the DAC (or whatever piece of hardware has the DAC incorporated). Since your Peachtree’s internal DAC does not unfold MQA you’d need a new DAC to do this. There are some reasonably priced ones that do MQA. See: https://www.mqa.co.uk/playback-devices

Since your Peachtree has analog inputs you can connect a DAC or Streamer/DAC’s analog output to it.

But if you want to try just the first unfold of MQA (24bit/96kHz max), then you have many more options, and maybe even some software on your PC could do that part I’m not that familiar with music playing PC software, so hopefully someone else might chime in on that aspect.

I had a Topping D90SE DAC that does MQA. I sold it and a few years later (last month) bought the $100 less Topping D90LE. Both are great DACs and a used one should be about $500. 

I also had a PeachTree Nova 150 and the DAC in that is not comparable to the latest iteration of DAC, including the low-cost Topping.

BTW - I ditched the MQA when the MQA guys gave a demo I was at, with about $200K+ of gear. I could not hear anything magical with MQA. I likely will also ditch my Qobuz and just use Tidal, so I have more than enough reasons to use MQA. I think my Qobuz is messing up somehow (or it could be something in my streaming chain).

I'll stick with QobuZ as do the majority at shows, no MQA please.