What you are describing is not really an "export".
It is more of a "print to file".
ok, how do I do it? How do I get a matte as the background and the image centered in it?
In the Library module, select the images to be "printed".
Click the "print" tab to go to the Print module.
In the Image Settings, select "rotate to fit".
In the Layout panel, adjust the cell size to the shape you are looking for.
In the Page panel, check the box for background color, pick the color you want.
In the Print Job panel, change the "print to" to "JPG file".
If I am understanding what you are looking for....
Will give it a try, thank you.