Unfortunately, getting the ladders to work like actual ladders is a bit beyond modder capabilities, mostly because of how flooring and traveling between them works. EA's system uses "portals" (yes, portals) to get sims to go between floor levels. It's why no one has done it and probably won't until the right conditions appear.
I had asked a fellow modder (whose taken apart this game and studied it) about ladders in early December since he's been working on animations lately (and attempted ladders before), and he said (copy/paste from our convo):
"The problem is getting portals to work.
It's how the game thinks that two places are traversable using one object.
If you don't do that, sims won't actually use that object to move between floors.
Portals have special tuning for how to animate in them lol
which needs a walkstyle or just plain old teleporting."
And walkstyles aren't able to be created at the moment. It's the same reason why modders haven't converted the animations of bikes for kids and why we only have stretching, and this is because bikes use a walkstyle, same as dog-walking, walking up/down stairs, etc. Like most things in this game (and real world), things that seem easy at first usually aren't.
However, he did tell me just a few days ago he's been working on cracking walkstyles, which if he gets this working means all sorts of mods may come out (I'm already planning strollers as soon as he finishes). I'll ask him again if this new, and hopefully successful, technology he's making will finally allow ladders, but I wouldn't bet on anything just yet.