Sure, but that's a) going to take some time and b) not going to be very convenient. Pulling something designed for a car (e.g. built in to the frame) and putting it into something for the grid are very different design spaces, so it could end up being prohibitively expensive to retrofit these car batteries into the grid system. Each manufacturer is going to use a different form factor, potentially different voltages, different cooling systems, etc. It's probably easier to break down the batteries and remanufacture them than to reuse them directly for grid storage.

What I do think could be a huge boon is to use cars at rest as storage. A lot of people leave their cars plugged in all day at work (peak generation), as well as at night (no generation), which is a pretty decent fit for a base level of supply. You'd basically drive to work mostly empty and get home mostly full, and you'd get a discount on your energy bill for allowing your EV to be used for energy storage. I don't know if any utility companies are using them that way, but that's a fantastic way to get a bit more use out of EV batteries.