Laravel Cashier - How to manage Generic Trials

In my app I'd like users to register and start free trials without entering any payment information. To do this, I'm using the Cashier's generic trial functionality, leaving Stripe completely separate from it.

Now, I'd like to allow users to be able to switch to a paid plan while on generic trial. Is there a way to add the generic trial remaining time to the Stripe's subscription expiration date ? Is there another way to handle this problem ?

Thanks