Of course there isn't an option to make custom pants, I think the first thing everyone would do would make someone naked and then troll little kids on CT with it xD
You can't make shorts. However there's a way to make it look like you are wearing a whole outfit with shorts/leggings. That is to use a dress design template -> black out the bottom -> color in the shorts and legs.
I made shorts/leggings with my designs. I made [pirate jacket] and [France's tailcoat].