This can be a little tricky, but you can take invisible platforms and change their pitch/roll angle. The best way to do this is to find a model that is a visible plane the same size as the invisible platform (I think there is one in Ogre's: Refuge in Ruins) and set it as the invisible platform's model. Now change the pitch/roll angle to a negative number by holding shift when you double click on the field you want to change. It'll take some trial and error to get the right angle. Now select the platform and raise or lower it with ctrl+pg up or ctrl+pg dn. Once you have them lined up where you want them you can change the model back to the invisible platform. Now your characters can walk up and down them.
Sorry for the long text, but like I said, it can be a little tricky. Good luck