Carry address over with customer's order when creating user
It's not implemented, but possible.  I think it should be part of Commerce core really, and done when the user is created by default.

In the meantime, my Register On Checkout plugin does this - https://github.com/bossanova808/CommerceRegisterOnCheckout

You could adapt this more generally I guess to listen to all user registrations and see if there are addresses to move over.

The actual code that updates the addresses is here: https://github.com/bossanova808/CommerceRegisterOnCheckout/blob/master/commerceregisteroncheckout/CommerceRegisterOnCheckoutPlugin.php#L312 ... but most of the init method would be relevant I guess.