It's better to leave those fields as is, since they are mapped to another sproc parameter and table column.
Another quick way of doing it is to directly update the field after the call to UpdateCustomer. Since at that point you already have a registered customer object.
C#/VB.NET Code:
DB.ExecuteSQL(string.Format("UPDATE Customer SET TestField = {0} WHERE CustomerID = {1}", DB.SQuote("test value"), ThisCustomer.CustomerID));
This mod you'll only have to do on createaccount.aspx.cs, assuming you'll only be displaying the input for that field on that page.
If that value is something you're gonna be using regularly, it's better to add it as another property on the customer object so you can access it more conveniently on other pages.
You will need to modify a couple of stored procedures for that as well (aspdnsf_GetCustomer*) and add that field on the result set and extract the value on the Init method of the customer.