I'm sorry to hear about the accident, but I would not attempt to make a claim
on an insurance policy purchased after the fact. Insurance is always bound for the current time and forward. Any claims that happened prior to the date of binding will not be covered. An attempt to get an insurance company to cover a claim that did not happen during its term would be considered insurance fraud
and could land you in legal trouble.
My advice would be to have the person who took the vehicle and damaged it pay for the damages. If they took the car without consent, the damage is certainly their fault. Going forward, I would recommend having coverage in place when renting
a car or driving your own vehicle. Even a non-owners policy
may provide some coverage in these types of situations.