Creating Fixed Data Dropdown in user profile

I want to use a database that returns industries and subs of the selected industry in user profile. So I need to create a field Fixed Data (dependent dropdown) for a newly created field in the user profile, but it appears it is only accessible in the fields of content type. User profile is not a content type.

Should I create a content type "user" first then create a Fixed Data (dependent dropdown) that selects from the industry database and somehow make that field available in the user registration form? Is this a workable solution?

Drupalgirl