Standard and workshops registry forms

I want to use a standard registry form for standard users but I want to create a checkbox in the standard registry form called workshops. If it is checked:

1.- The user is not approved and he received an e-mail informing him that to be approved he must send documentation through email. Upon receipt of the documentation, when I approve the user he is automatically registered in the workshops group.

2.- The user is not aproved, he sees a message informing him that to be approved he must send documentation and just below he have an option to upload the required documentation. Upon receipt of the documentation, when I approve the user he is automatically registered in the workshops group.

Is this possible?

Is this possible otherwise?

Thanks.

I wanted to use the default hikashop register form if it is possible. I see that I can agree custom fields to that registry form son I thought maibe it could be done.

But from what you say in your answer Ithink is not possible so I have to use another extension.

Thanks for your answer.