Do you mean taking an existing ICE frame and making an EV version? It happens. The Mini Cooper EV is a Cooper S with the guts from the BMW i3 dropped in. They changed as little as they could get away with. They even left the hood scoop on.
Meaning it can be done. I don’t think there are that many engineering challenges to overcome. If the main obstacle is money, perhaps stopping the subsidies to Big Oil would help?
The main obstacle is that they aren’t very good. They’re a transitional step. We’re already moving past the point where it makes sense. The next Mini EV models coming out will be purpose built designs.
It can be done, but it makes a worse product. EVs are built to fit batteries and motors in the most optimal place. Likewise with ICE cars with engines and transmissions. What you end up doing is shoving batteries in the engine compartment which is shaped wrong and you significantly change the balance of the car. You leave much of the expensive parts of the ICE car, while adding more expensive parts. It just doesn’t work well in practice. If you are going to spend time engineering, it is better to engineer a proper EV than try to shoehorn an EV into a size 6.