Best air conditioner for cold blasts?
The change is we are regulated in some cities and or states to energy star requirements, if I put in a massive system I can get in trouble, not true in all areas but this may be why. Can you have a split added to make 1 room a “freezer” sure. 
Call your local HVAC company and tell them you want a split in your room that you want cold, tell them you have poor insulation and how many square feet and the number and size of the windows, and ask what is the size unit needed to really cool this room down (some controllers are limited to 68f) and let them know what your desired temp is for this room, if they can do it they would love the work, that or you get a massive window unit and put some insulation on the temp probe. That’s kind of how I made a walk in cooler in my garage using a portable ac unit.