I want Creep Tumors to always generate Creep on the Ground Level 1 (there's 0, where you can't place units normally, 1, 2 and 3).
If CT is placed on 0: creep generated for level 0.
1: 1
2: 1+2
3: 1+2+3

Get it?

I believe we have to mess with the Behaviours ;o

I would assume that creep is modifiable, since it was in the wings of liberty mission so i would assume that behaviors you would have to apply the behavior to the current level by detecting where the creep stops you can detect the next level and apply creep (waterfall creep ) down then that would run a trigger or behavior which will add another layer of creep hard to explain its basically a water behavioral physics engine is what your designing...

so if the creep is on lvl 2... when it reaches the lvl 1 cliff it detects it as a lower level cliff so it knows to move the creep down... when you get to lvl 3 cliff it detects it as higher so it won't move up... is this what you want?

Yes.

Try looking at reapers behavior that allows them to jump

You're probably right, though I will not implement this in the end, for the purpose of keeping game's normal settings: it's an impossible scenario after all... nothing must be changed!