How can an address database be converted to an account profile? I imported an address database into Drupal as a content type and I want to convert into an account profile, with users having the ability to make changes it, and use some of the fields in it to link to other content. Are there some modules which can help accomplish that?

At the very least, after creating a Drupal account for each entry of the database, I should be able to link the user account to a particular record of that content type, or import some of the records fields into the profile.
If you want to keep current content type, add Reference field to your user and reference your type via reference field.

If you don't want to keep current content type, it is better to import the data directly into user fields.