SharePoint doesn't manage its own "Accounts". It authenticates against a configured authentication provider either using Windows Integrated against Active Directory or Forms-Based against an LDAP store, SQL Membership & Roles, ADFS, etc.
If your legacy application used a custom account store, then the closest architecture would be to configure SharePoint for Claims authentication, configure forms-based authentication, and use the SQL Membership and Roles authentication provider. You would need to write an import tool that creates accounts from your custom CSV file and creates accounts in the SQL Membership and Roles database. You could also choose to implement a custom Membership and Roles provider against any data source you choose.
For additional user attributes, you would either need to write custom code to augment the claims token, or configure user profile synch with BCS to map user profile properties to a custom SQL database, etc.
Sorry, but your solution is not going to be as straight-forward you as are implying. Authentication, User Profiles, User Profile Synch and Property Mapping, and MySites are all essentially different moving parts.
To accomplish this would require stitching together a variety of blog posts and MSDN articles depending on the specific architecture you choose.