Hey no problem.  We're all learning at one stage or another.  Lets actually get to the root of what you are trying to accomplish because maybe this can be achieved in a completely different way.  It sounds more like you are trying to authenticate the user against an external database + DNN database credentials so what we might be looking at is a customized Authentication Provider.  It'd be great to hear what your end-result needs to be and then perhaps we can offer up some suggestions/solutions that work well with your experience and leverages 3rd Party modules (if necessary).

Okay... I see this being possible in one of two ways:

1. Custom Authentication Provider
2. Custom Login and/or Registration module

Lets talk about the two methods and how they might be accomplished.  Using a custom authentication provider is probably overkill because for the most part you still want to use everything in the DNN authentication provider with exception that you want to add a step.  You'd still need a way to provide for entering in that ID upon registration and then your authentication provider could use that value behind the scenes and do whatever it needs to do.

Alternatively you could go with a custom login/registration module.  I like this approach because you can hand-tailor your registration module to be formatted the way you want, ask for the fields you want, etc.  You could still use *all* the DNN code behind the scenes to accomplish the registration and that is good.  What I'd do is have it fire off the code to check the ID upon registration and validate it... if it is good, allow the registration to complete and save their ID as a custom profile field.  In a nutshell, here is some code (copied straight out of my own production code) that saves a new user and stores something in their profile.

DotNetNuke.Security.Membership.UserCreateStatus crStat;

// add user first
DotNetNuke.Entities.Users.UserInfo info = new DotNetNuke.Entities.Users.UserInfo();
info.Profile.InitialiseProfile(this.PortalId);
info.DisplayName = textDisplayName.Text;
info.Email = textEmail.Text;
info.FirstName = textFirstName.Text;
info.LastName = textLastName.Text;
info.PortalID = this.PortalId;
info.Username = textUserName.Text;

info.Membership.Password = textPassword.Text;
info.Membership.Username = textUserName.Text;
info.Membership.Email = textEmail.Text;
info.Membership.Approved = false;

info.Profile.PreferredLocale = "en-US";
info.Profile.FirstName = textFirstName.Text;
info.Profile.LastName = textLastName.Text;
info.Profile.SetProfileProperty("Employee_ID", textEmployeeId.Text);

crStat = DotNetNuke.Entities.Users.UserController.CreateUser(ref info);

So now lets assume you have that.  If you have your own login module you can validate the login with the ID (load the user and then use the info.Profile.GetPropertyValue("someproperty") to retrieve it).  You can use DotNetNuke.Entities.Users.UserController.ValidateUser to ensure credentials are correct and, if so, then use DotNetNuke.Entities.Users.UserController.UserLogin to log them in (and then you should redirect them to a new page or the same page so the cookies get loaded in and used).

Hope that gives you a starting point.  Any more questions, ask away.

DotNetNuke.Entities.Users.UserInfo info =

You can cast string to bool but it needs to be a compatible cast (ie the string says True or true, False or false).  Does the ValidateID return an empty string if the ID is valid?  If that's the case you could merely check that ValidateID = String.Empty although I'd be more inclined to see the ValidateID return bool and have an output parameter that contains an error message when/if there is an error.  You can use Boolean.TryParse to do a graceful cast to bool from string or you can use Convert.ToBoolean for a more direct cast.

I have some complete code that I'm using for a simple registration module if you'd like.  Contact me directly and I can zip it up (it is part of a larger module so i'll just zip up that control and code).

*EDIT* Sorry, forgot to answer your question about whether you can just use the code and modify inside of the User.ascx.vb.  The short answer is yes, you can modify that code if you want.  The problem with that is whenever an update to DNN comes out and you want to upgrade you'll have to reinject your changes into that file.  That just gets nasty.  It is much easier IMHO to create a module that extends the functionality you're after instead of trying to change the core.