I mean do I need to insert any record to other tables so that system integrity is still kept? Please help. Thanks.

It's best not to manipulate the database directly.  If you want to add users programmatically, do it the right way:

$new_user = new ElggUser();

$new_user->username = 'new_user';

$new_user->name = 'New User';

$user->access_id = ACCESS_PUBLIC;

$user->salt = generate_random_cleartext_password();

$user->password = generate_user_password($user, $password);

$user->save();'

To generate metadata, say:

$new_user->metadata = "Value";

And if I need to set the same widget to all profiles? There's a way to do it programmatically too? I mean, I have created a news plugin to be showed when the user login, and I want to set it to all users.

@Crisdev, use the default widgets plugin

Isn't it to new users only?

Yes the default widgets plugin won't work for existing users, only those who register after you've saved your default widgets settings.