you can just get a sealed box or a customized sealed box for the sta.fe trunk.

and YES you can have you car setup BOOM BOOM without changing the HU.
this is my current setup in our everest.

so you will need an amplifier and a sub woofer with a sealed box at the back.

the wirings can be tapped from the hu to the amp and the amp will supply the seps and the sub. 2ch. each if the amp is 4ch.

hth goodluck on your project.