l shape functioning stairs

I have been researching this as well.

It is possible to make the stairs in Blender, but you would have to use a deco item to use to get the stairs into the game. The stairs would not be functional. S4S doesn't have the stairs in Build mode.
However, you could create a mod that allows the sims to move up and the down the stairs. This I would not know how to do, but there are tutorials here on animation and tuning. 
I saw on twitter that a guru posted a poll on what kind of stairs to have in the game. The percentage for spiral stairs is much higher. So, they might be working on this; who knows for sure. If the spiral stairs are put into the game, then when creating the L-shaped stairs, to put them into the game you would use the spiral stairs, but then the sim will be using the spiral stair animation.

If you are talking about using the already ea stairs, there are plenty of tutorials on youtube to create these.