Create a new user from an entry submitted with the module "webform"
I am also looking for a similar solution and would like to reuse an existing one.
To assign a role on registration I would try some of existing modules.
Let me summarize what is available:

Auto Assign Role (D7, D6, D5)
Registration_Role (D5, D6)

To allow a user to choose a role on registration

User-Selectable Roles (D5, D6)

To provide a role using a key during registration

Registration Codes (D7, D6)

Sold Memberships

Ubercart (D7, D6)

Join role with a password

Join role with password (D7, D6, D5)

Apply for role

Apply for role (D7, D6)

Webform register

Webform register (see Open Issues for details)

See also integration Webform and registration module.