Just insert it using File > Insert Clip file.
Click on the Objects tab and you should see it there (Background_1) along with the Planet, and the original Background.
You can now delete the original Background.
To make the starfield more visible do things like:
Decrease Bluesky Density; Darken Bluesky horizon colour; Reduce Haze and Bluesky Expiry height; and Reduce the height of the Ceiling.
Then just play with the parameters of the various nodes of the clip file like the Space Dust Colour/Amount/Breakup. The amount of Medium and Small Star Clumps is adjusted with the Colour Offset parameter. And the Colour Adjust shader adjusts the brightness of the stars.
Hope this helps.
John
