Do people who mod extra long mission or quests on games like Skyrim ever tried selling those mission and quests for DLC and become game makers and video game millionaires themselves

Trying to sell Skyrim mods would be totally illegal, as there's always some part of it that you don't have the rights to. The creation kits you'd use typically do mention this in their terms and conditions, most of the assets you are using still belong to Bethesda.

Your only hope would be the parent company notices your projects and offers to officially license them.

Such examples would be the Half-Life mod, Counter-Strike, which was purchased by Valve, and Bukkit for Minecraft, the creators of which were hired by Mojang.