integration Webform and registration module
There are two contributed modules that may provide what is needed (quotes included are from their project pages):


Webform register:


  ... adds a Webform component that collects user registration details and creates a new account. This is useful when you only want authenticated users to submit webforms, but you do not want to create an extra hurdle for people before they fill out your webform.
  
  Webform Register can help you keep the flexibility of Webform, while reducing duplicate entry for your visitors. Because the user registration page can be extended through Drupal's built-in user profile fields or with the help of Content Profile, the Webform Registration can help you collect user-specific, oft-repeated information such as Name, Address and Organization information and store it in a profile entry for each visitor. Meanwhile, information that's only pertinent to the individual webform, such as survey data or preferences for a single event, are collected by standard Webform behavior.

Webform Registration:


  ... born from the need to allow visitors that came to the site to fill out a sweepstakes from which needed to not only submit values to Eloqua, using the eloqua module, but also create a new user.
  
  At the time the Webform register module did not provide enough flexibility for what we needed to do. Which was have many different field names (used for eloqua mapping) and still keep track of which ones held the values for email, username and password.
  
  How is this module different from Webform register:
  
  
  Maximum webform integration and flexibility
  You can add and fully customize every field that will be used for registration.
  If you are in a hurry, just add en email field and this module will handle the rest via a very user friendly and intuitive interface.
  Only the email field (webform component) is required, all the other components can be either generated by this module or omitted depending on your needs.
  Optionally you can specify all the labels and descriptions for all five fields (components): email, username, password, password confirmation and email confirmation