No, don't link nothing to the 'users' copy of Joomla native table.
Deals with Cook, as if you didn't have it.
Then, in your component, you'll get MVC to manage a 'users' table (just change the name inside table file)
This is the first step. With this MCV source code, you'll have all you want to manage and hack '#_users' DB.
In your form, you cannot insert users native fields because are behind FK.
So, in your form template source code, add some inputs (HTML, or JDom, as you want),
And finally, in the controller, do all the job to split the datas when storing.
Of course it is long to explain all the work you have to do. This is only a pattern. Then you will have to write.
I wrote a long post specially for you, and when I posted it, when session finished, I lost the text.
So boried to have to rewrite, I gave you a short answer.
Maybe tomorrow, I give you more precise help.