Custom registration form, profile import

Greetings,

I was reading about custom profile import fields on the following URL:
www.sourcecoast.com/forums/jlinked/jlink...e-fields-into-joomla

In that it is mentioned that: "It's also possible, if you have a developer, to create your own profile plugin which can simply store data in a custom table in your database. Not overly difficult, but you'd need knowledge of PHP and MySQL to do it. "

Could you possibly point me in the right direction on how to achieve this? I have LinkedIn login working successfully but need to achieve the following:

a) Override the default JLinkedIn registration form to allow for additional fields
b) Retrieve the LinkedIn profile fields to automatically populate the over-ridden registration form

An example of the additional fields I am trying to capture can be found at www.odysseytraining.com.au/create-an-account/, saving the fields to the custom profile I can handle but not sure where to begin on customizing that form and retrieving linked in profile information,

Any help you could provide is greatly appreciated,