Here is the procedure to follow to create your own Sky-System to use with the Extends plugin commands.
Proceed as follow :

1 / Create a simple text file withe .x4r at the end (file extension)

2 / Create a drawer with the same name than the previously created file.
(you must use the same name than the file but without the .x4r extension)

3 / Open your freshly created drawer and create your textures :
A - blue texture for the sky color in full day (12h00).
B - Clouds texture.
C - Stars texture.
D - Texture for a halo around the sun when he go down.
E - Same than 'D' but for when the sun go up.
F - Grey texture for sky fog (named gris.bmp).
G - Ground texture.
H - Light persistance for when the sun go up and down (sky illumination).
I - moon texture.
J - Texture for far backgrounds (mountains, cities, etc...)
K - Sun texture.

4 / Copy these 4 files from an original SkySystem into your drawer :
Ciel.x
FarAway.x
Ground.x
LC.x

5 / Open your .x4r file with notepad or wordpad :
Change the lines written in lowercase to make them fit
with the names you given to your textures.
(excepted for the first line : X4 RealTime-SkyBox Definition that must remain unchanged)

X4 RealTime-SkyBox Definition
CIEL:
Sky color (A)
NUAGES:
Clouds textures (B)
ETOILES:
Stars texture (C)
SOLEIL:
Sun texture (K)
LUNE:
Moon texture (I)
HALOLEVER:
Halo for wake up (E)
HALOCOUCHER:
Halo for sun going down (D)
AURORE:
Light persistance around sun (H)
BACKGROUND:
Background texture (J)
GROUND:
Ground texture (G)
ENDOFRSD

6 / Copy the .x4r file and the drawer into your project drawer.

You may then be able to use your own sky system in your creation.