The HDD from the Egreat should work in the Zidoo with no modification. You'd then need to add the source in Zidoo Home Theatre. Add a source using the drive icon at top right of Zidoo Home Theatre.
If you want to add a second internal 3.5" HDD to Z20 Pro you'll need to attach it via USB in a dock. You would then add this drive as source in Zidoo Home Theatre the same as above. There are many docks available for different numbers of drives, but in general any USB dock will work. E.g. https://www.amazon.co.uk/FIDECO-Docking-Station-Support-Offline-Black/dp/B09X9WN96L