Adding simple registration form

Hi, I’ve been using this amazing plugin for a while now, but there is something that I would like to achieve but I don’t know how to do it:
I want to put a simple registration form at the bottom of my forms with only three fields:

Username
Password
Confirm password

And I want to be able to create this new user at the same time that my form with all his fields is submitted.

Do you know how I can achieve this?
Thank you
(sorry for the poor english)

http://wordpress.org/plugins/wordpress-form-manager/