In theory, yes.
In practice, the cost & hassle of the necessary wiring and control circuitry would seriously restrict the appeal of this.
Some Tesla owners in Texas have been sleeping in their cars with the heat on. The new heat-pump models especially should go for quite a few hours that way. No risk of carbon monoxide poisoning, either.
The electricity hasn’t been down continuously, either, just for long periods and often at inopportune times (like at night). So the EV has an opportunity to recharge when the power is available and can run on the battery otherwise.
EV critics like to concoct extreme scenarios where they don’t work well (while failing to imagine scenarios where fossil fuel cars have problems). But we have a real disaster situation happening right now and EVs seem to be handling it just fine and are even proving to be a boon.