Drupal 7 : i want to add the registration fields (username/e-mail etc) to a profile 2 registration form
I have achieved the above with a similar setup to yourself.

You state you have used 'registration wizard', so I assume you have created custom forms for the profile2 fields. Furthermore, you are validating and submitting the values at the end.

The simplest method is to create a custom form at the beginning of your process for username, email, password, etc. Then using the user_save() function. Of course you will need to ensure current users don't exist already, which is easy enough using user_validate_mail().

Hope this help.
Amarjit