Open the model in Inspector and in the view pane highlight the part of the model that will move up and down. Find the corresponding node that's highlighted in the scenegraph tree. Right click on that node and select 'insert above' and then 'transform' and give the transform a name. Go to File > Save As to save a new version of the model with the transform node in the scengraph. See the
hierarchical models page for example code to get a handle to a transform node in the script. You can move the floor up and down by moving the transform node.