you should not modify mojoPortal code, otherwise you will not be able to upgrade.

If you need to do custom development you should work from the source code, the .zip files are meant for production deployment and have no C# source code, it is all compiled into dll files in the /bin folder. To work in Visual Studio you should get the source code (but not modify it, only use it for learning and example code). See also Understanding The Difference Between Deployment Files and Source Code, and Avoid Forking The Code.

You can add custom fields by configuration, if you need to run additional logic after user registration you can implement a UserRegisteredEventHandler.

If that does not give you the control you need then you could implement a custom registration page in your own project and use a post build event to copy it up to replace the mojoPortal Register.aspx, but then it is up to you to implement all the registration logic.

Hope it helps,

Joe