You can use the DotNetNuke.Services.Mail.Mail.SendMail(...) function to send email to a user yourself, or you can even "manually" send email using the built in .NET mail functions (System.Net.Mail.SmtpClient).

If you want to see the code where it creates a user (currently), check User.ascx.vb "CreateUser" method and see that it fires off some events (OnUserCreated).  That is in UserUserControlBase which is defined in DotNetNuke.Entities.Modules.  The OnUserCreateCompleted event is handled in admin/Authentication/Login.ascx.vb line 909 (in my dev copy 4.7 so could be a different line) -- function is UserCreateCompleted.

You'll see in there that it has a strMessage = CompleteUserCreation(e.CreateStatus...) which, inside there, finally calls the DotNetNuke.Services.Mail.Mail.SendMail function.

Hope that helps.