Is there any cheat/mod/way to create more floor levels?

I'm building my own apartment (the one I live with my parents), but the game only allows me to build 5 levels, and the building has 8 (well, it's not necessary, I can build it with just 5 levels, but I would like to make it as real as possible)

The short answer:

No, there's currently no way to do this.

Long answer:

The answer to your question is actually yes and no.

"No" in the sense that only 5 above ground floors may be usable, but a maximum of 4 basement levels. Also, every time a foundation or frieze is used, that will "use" up one of the floor slots, and of course those floors won't be usable either.

"Yes" in the sense that using shell buildings and CFE can give the appearance of more floors, but only 5 at most are actually usable. However, there are downsides to both the shell and CFE methods.

Shells aren't CASt-able, and in my opinion, I think the standard EA shells aren't that great, and really only might fit in certain towns/worlds. So to get different colors require making custom shells, and this becomes object making more so than actual building. Also, I don't like the shell buildings so I haven't used them, but I've read that once one of them is placed, you can no longer move the shell or even delete it from the lot individually. The only way to remove it is to bulldoze the lot, and thus everything on it.

One can also use the CFE method to achieve tall looking buildings, and a few world threads in CFF show many examples. However, one thing they all have in common is the fact the stretched walls only have wall textures, no windows. Windows remain the same height regardless, and you can't stack windows on the same floor. So while it's not impossible, it's more difficult to make such buildings look good.

You can build higher than the five floor limit using cheats. http://www.modthesims.info/article.php?t=360606

Another approach is to make deco buildings like the ones in Manustyle's New York World. These buildings are limited ony by your imagination. You can make them as tall as you want -- 1000 stories if it so pleases you, You can build wall on funny angles or even make round buildings. The downside is they are purely decorative and sims cannot live or work in them.