Do you want the stuff to grow from a ceiling? or on walls.
If you want it to grow down from the ceiling/roof then just use them in your ecosystem.  In the density tab, choose offset from surface(bottom left) and change the variable to a minus number.  It depends on the size of the model and the scale you have it set at, but what you will be doing is growing it through the floor.

If you want it growing on walls that's a little trickier.  

1. You have to rotate the wall so that it is flat--parallel with the ground
   2.You will have to go into the scaling/orientation tab and choose direction from surface and change that variable to 100%
2. Grow your vegetation then rotate the wall in place so that it is in its original position.  But once you change or regrow the ecosystem, it will get all screwed up unless you place the wall back flat on the floor and grow it from there.

Hope this helps.