You can sort of do that now by buying a gas-powered car and a conversion kit. It just tends to be expensive, and the kits don't always include batteries and battery boxes.

Example: https://www.evwest.com/catalog/product_info.php?cPath=40&pro...

I wish EV tax credits applied to conversions and not just new vehicles. It would make conversions a lot more economical, and maybe we'd start seeing low-cost mass-produced conversion kits designed for popular vehicle models.