Adding an external code or a better way...

Hello All,
I have a custom module that allow a user (manager role) to choose between manual or automatic.
If the user choose manual, I want to give him/her a list of users that registered, than s/he can drag&drop to a bracket and save it to the drupal entity "category".
If the user choose automatic, drupal will give her/him a list of choices to select from and than lottery the registered users to the right place in the bracket.

I have created a manual system that do it but it is external to drupal (php, html, css, js).
My question is how do I do it? either buildin in drupal or all for some kind of iframe to the external code to do it?

Any ideas? code examples?

Thank you ahead (the system here found me as a spamer so I couldn`t say thanks to thus who helped before).