Electrical engineer here. People used to do this for Prius Plugins and the occasional gen 1 LEAF. It's not impossible, but the complexity is astronomical for modern vehicles, and any practical solution for cramming an extra battery in there will be kludgey to the point of being more worthy of a $3k rustbucket than a $35k car.
- The onboard charger would have to be replaced or revamped. A battery pack is made of dozens to hundreds of cells, each of which has to be monitored for charge balancing, typically done by the on-board converter.
- The firmware would have to be jailbroken and reverse engineered. The car's computers need to recognize the SoC% and total capacity to report range estimates and to know when to activate the engine and hydraulic brakes. If you don't wire your sensors in right, or rewrite the right parameters, expect nonstop angry bees.
- As KentuckyKen mentions, the glycol lines would have to be rerun, and the pumps possibly replaced. The temperature sensors would have to be moved, and the controller recalibrated for the decreased cooling efficiency.
- The suspension would have to be reinforced. This is easily 500lbs of equipment we're adding here.
There are a number of cleaner options here. Do you want a bunch more EV range? Find yourself a BMW i3. Do you want 10 more miles to make it back from work? I suppose you could put a Yeti 3000 in the trunk and use it as a mobile recharger.