For info on how to create your classes, read the pinned posts at the top of this forum on how to create a module.  As for relational data, it's no different than any other method to get data from mulitple tables.  Just write your stored procedure so it access all the tables you need; they don't even need to be in the same database so long as there's some type of relationship.

SELECT
    'username' = u.username,
    'firstname' = u.firstname,
    'lastname' = u.lastname,
    'hiredate' = e.hiredate,
    'department' = e.department,
    'jobcode' = j.jobcode
FROM dotnetnuke.dbo.users U
JOIN companydb.dbo.employees E ON e.employeeid = u.username
JOIN hrdb.dbo.jobcodes J ON j.employeeid = e.employeeid
WHERE u.username = @username