May be too late for the primary ballot, but a VP choice can come from anywhere...
I think she’s too late for the NH, SC and FL ballots unless the states make an allowance for her. (Or she wages a vigorous write in campaign.) But they all award their delegates proportionally, FL is supposed to be losing half of their delegates, and she’d certainly have lost to Mitt in NH anyway.
It’s late, but she’s still got a chance.