Not at launch. Eventually he will be sold separately just like Goro last time, you will just have to wait awhile. So, if you really want Shao Kahn in a timely manner you should preorder
Is there any other way to purchase Kahn other than preordering the game. Can he be bought in ps4/xbox store?
With how things are today you might be able to purchase him day one. I preordered the shinier expensive REmake 2 and everything that came with it you could buy on the market on the day of release.
If you need to play as him I guess, and it really sickens me to say this, preorder.