Cloning features only applies to features that plug into the content system and is a way to modify them without losing the ability to upgrade.
I don't recommend making any modifications at all to mojoPortal code, if you do that you have created a fork and it will be difficult for you to ever upgrade. All of your custom code should be in your own projects.
You could implement redirect logic upon login in a user sign in event handler.
You should not be adding properties on siteUser nor adding columns to the table or modifying any mojoPortal code. Instead you should create your own table for those custom fields and create your own object like userExtensions and add your custom properties to your own object and populate it with your own code from your own table. You can keep your extensions associated to the mojo SiteUser by the userid or userguid which you store in your own table. In your custom sign in handler you get a reference to the siteUser then you could use the user id to lookup your own custom object.
If you really need to customize the login page (which you don't in this case in my opinion), you could create your own custom page in your own project then use a post build event to copy it up and replace the mojo version of the page. This would mean that you always have to deploy your own custom build but it keeps you from modifying mojoPortal code so you are able to always pull the latest change sets from the mojoportal source code repository.
Any time you find yourself about to make changes in mojoPortal code you should stop and try to think of a creative way to accomplish your goal without doing that. The solution will vary from case to case depending on what you are trying to accomplish and you are free to ask here and explain what you are trying to do and we "may" be able to help you come up with an idea.
Hope it helps,