I would advise against doing this after application submission.
This would imply that your organization (present or past) issues customized reference letters 'on demand' which discredits the organization. This would only serve to make the officer suspicious about the legitimacy of your work experience leading to extensive background checks. That might still be alright but imagine an officer's reaction if your HR/supervisor tells them that they provided a customized letter on your request (or worse, that they just signed a letter written by you!). At best, you are assured of a potential refusal. At worst, a misrepresentation charge as well.