I do have the XM reciever unit and an account, do I HAVE to buy the accesory, or is there a way to add my account to the unit to make it work?

Which "xm receiver unit" do you have? If its not a Pioneer, its not going to work. You need theirs

I have the XM Express XMCK-10a, and I may be an idiot but it does have a car install, to install this do I have to have the installer pull the wole head unit out again or is this something I can do as a begginer?

Sure, it'll work with the pioneer, but you're not going to be able to use the D3's XM radio interface. If I were you, I'd sell the XM radio you have now, buy the Pioneer tuner module, and transfer over your XM radio account to the new tuner. Because using the D3's screen for the XM radio > just about anything else.