Would it be possible to have a complement that allows the user to manage the register?

Our organization needs a form that allows the user to update the register with a username and a password 

Hi,

It is possible to allow your users to return to their form and edit their submissions.  All you would need to do is include an edit link in your in your autoresponder:

http://www.jotform.com/help/40-How-to-Let-Users-Update-Their-Form-Submissions-at-a-Later-Date

Though it is not possible to password protect this feature, the link that will be placed in the submission response will be unique to the specific submission.  The user will only be able to edit their own submission and not others.

If you have any further questions, let us know and we will be happy to assist you further.

That's great!

how could i know the edit-link for forms that are already registered?

It is possible to send edit links for previous submissions, this would need to be done manually however.  To create the link, you need the form ID# and the submission ID#.  Both can be find in your submission view.  You would then create the link as such:

http://submit.jotform.com/form.php?formID=xxxxxxxxxxxxxx&sid=xxxxxxxxxxxxxxxxxx&mode=edit

The form ID will go after "formID=" and replace the x's.

The submissions ID will go after "sid=" and replace the x's.

There is unfortunately not currently a way to automatically send these.

Let us know if you have any further questions and we will be happy to help.

Better than nothing, it works. Thank you so much.

You are very welcome.  Let us know if there is anything else we can assist you with.