Paid Member Subscriptions doesn’t offer a built-in way of customizing the registration form, but this can be done using Profile Builder since the two plugins are integrated with each other. Profile Builder is a user management plugin that lets you customize the registration form with an unlimited number of custom fields.
When both plugins are active, inside Profile Builder -> Form Fields you’ll find a new type of field that you can add to your form: Subscription Plans. With this field you can let the user purchase a Paid Member Subscriptions subscription plan from your Profile Builder form.
You can read more about the integration on this page.