The checkout process is one one page so you can't get in between the checkout steps but you can do that between the basket and the checkout page. For that, create a named route to your login page and configure that name for the checkout target in your ./config/shop.php:
If the customers have logged in or are already logged in, redirect them to the "aimeos_shop_checkout" route so they can continue with the checkout process. If you don't want to allow guest checkouts at all, you can protect the "aimeos_shop_checkout" route with the "auth" middleware.
Help for integrating the Laravel package
Forum rules: Always add your Laravel, Aimeos and PHP version as well as your environment (Linux/Mac/Win)