Can I introduce a DAC to this??

Agree with the above. scott-nixon.com even includes a tube output, high value DAC among its custom designs for sale. Prices are very modest for the performance gains you would expect to hear.

Does it have a digital output (coax or toslink)? If so, yes you can use a DAC for much better sound. The other advantage of an external DAC is that you can play around with different ones to get the sound you like.
Also, if the Pioneer breaks, which changers often do, you aren't stuck with a unit with expensive mods.

If it has a digital out (either coax or optical) then you can add a DAC. There are some really good affordable DACs available today. A used MSB Link DAC II or III, Perpetual Tech. P3A, Channel Islands, Birdland Odeon Lite, Scott Nixon, dACK!, etc., would likely be far superior to your changers internal DAC.

Enjoy,

TIC