This won't make it endless. It'll just make the area seem huge. For the scene flag, you first need a block with a sky texture assigned to it then define this entity as a sky object.

Code:

---

sky some_sky
{
	... // sky definition code goes here
	flags = scene, visible;
}

action move_with_camera
{
	some_sky = me;
	
	while(!some_sky)
	{
		wait(1); // wait until pointer is valid
	}
	
	while(1)
	{
		my.x = camera.x/16; // play with the 16
		my.y = camera.y/16;
		my.z = camera.z/16;
		wait(1);
	}
}

---

You may need to ent_create the entity to get it to work. For infinite distance, instead of setting the my.x-related stuff, use vec_set(my.x, camera.x) to copy the positions of the camera. This way, the region is infinitely distant all around.