Wither skeletons in the game code are actually regular skeletons marked with additional 'flag'. That 'flag' is checked in the code to change their size, weapon and drops. That flag is initialized when the skeleton is created - it is always 'regular skeleton' if the skeleton is created in the Overworld; and it is 'wither skeleton' 80% of the time if the skeleton is created in the Nether.
So, there is no way to spawn wither skeleton in the Overworld in vanilla Minecraft; not even with commands.
But you can always spawn a wither skeleton in the Nether and push in through a portal to the Overworld.