mmm... no idea bout that, sorry. BUT...
1) Import the moon texture photo into your sky/galaxy photo. Make sure it's above the sky/galaxy photo layer.
2) Create a circular selection within the photo where you want the moon to appear.
3) Add a Layer Mask to the layer.
Done.
If you should want to move the moon around in the image, do one of these:
1) Choose the Move tool, and click hold & drag on the moon layer around.
2) Click on the chain link between the layer and it's mask. That disconnects the two. Now click on the Layer Mask, and using the Move tool, drag the circle mask around the photo.
If you wish to,
although i don't recommend it, you can add another step after #3 -- under the SELECTION menu choose "Feather" and set it to 2-3 pixels.
That should do it.