Black Bar Plugin Development

Okay, maybe it is not called black bar, but you all know these sites where you are looking at different theme's for wordpress maybe of these has this black bar in the top with is always "on top", I want one of those for a project im making right now.

I'm aware that I properly need to get a developer to make this, but before I try to find one who will do this I want to know if it is even possible.

So right now I have 3 sites x.com - y.com - z.com, they are all running on a multisite, and i'm using Domain mapping plugin to have its own site name. I'm using multisite user management to share the same users.

Now I want this black bar in the top, where I have a link to the 3 sites, and the in the right side I want (Login) - (Register)

When you are pressing on Register I want to open a lightbox kinda windows where you can then register (Username) - (Password) - (Email) and then press register and go back to where you where.

The login has to do exactly the same, and when you login while you are on x.com, I still want to be logged in when I go to y.com

When you are logged in, I want link to a (edit profil) and then again a lightbox to edit some basic user settings.

The users are just logged in user, no need to edit anything.

I want the admin bar to dissapear as well of cause, but I guess that is not the biggest problem.

Now my question is, it this even possible, and roughly how much would such a "plugin" cost ?