Skymaps have to be saved into your map config. So open up your maps folder find the right .cfg file and find these lines; I'll just copy and paste for you to see.
Now since yours is custom, make your own path to the file and delted/comment the default skymap. This is at least how you used to do it, pretty sure nothing like this can be done ingame still.
EDIT: Woot. Beat you Andrez.
Code:
// The skymap for this map. You can change this by commenting the one below, and then uncommenting one of your choosing
// Alternatively, you can add your own into "packages/textures/skymaps" and use it as below.
loadsky "textures/skymaps/steini/steini4" // Comment this if you want to use a different skymap.
// loadsky "textures/skymaps/steini/steini2"
// loadsky "textures/skymaps/steini/steini3"
// loadsky "textures/skymaps/makke/rocky"
// loadsky "textures/skymaps/makke/mountain"
// loadsky "textures/skymaps/iceflow/iceflow"
// loadsky "textures/skymaps/ladynighthawk/inhcanyons1"
// loadsky "textures/skymaps/socksky/nightball"
// loadsky "textures/skymaps/socksky/grave"Now since yours is custom, make your own path to the file and delted/comment the default skymap. This is at least how you used to do it, pretty sure nothing like this can be done ingame still.
EDIT: Woot. Beat you Andrez.