As far as I could figure out, there are two ways to "hack" L-shaped stairs but these methods are not very satisfactory:
Version 1 - basically you make a stair going up to the second floor, another staircase, rotated, going up to the third floor, and delete the second floor, so you have to skip the second floor, your stairs will be really long, and your builds will have really tall walls/ceiling heights.
Version 2 - you have a house with a foundation, and for part of a room, you delete the floor over the foundation, then make stairs going up, then make more stairs, rotated, going up to the second floor. The only problem is you can't have the front door of the house starting at the bottom level, it has to be at the level of the foundation, so you will have to have a "sunken room" or split level to achieve this. In other words, open the front door, go down a few steps, then up an L-shaped staircase - not really workable for my purposes.
If anyone knows a better way right now to achieve this I'd love to know. I looked around and couldn't even find any custom content L-shaped staircases for Sims 4.