Admin can define custom user profil fields providing you install the Predefined Attributes 0.3.3 extension (by michaelontheroof).

You can also add custom fields to the registration form:

• Extended Application Form 1 by Mark: “extends the application form so that the user's first and last names are on the form and are required…”

• Applicant Discovery 1 by Mark: “Adds a “How did you discover this forum” question to the application form and saves it to the database. The response is emailed to applicable administrators along when the application is complete…”

• The Vanilla Documentation on "Delegation" provide as well a tutorial to create an extension that add custom user fields to the registration form.