Your question is a little brief but one option you have is that you can drape your data over a digital surface model (DSM) to extract heights. In the past when I have done this I have draped the features that I have wanted to add heights to over a DSM and then taken an average of heights extracted for the (in my case) Building and then used the 3DForcer to create 3D buildings.

You can also try the Extruder transformer to convert the building footprint from 2D to 3D, once you've got the Z values as 1spatialdave described. Have a look at the KnowledgeBase article on extruding building outlines . There's also a tutorial on 3D modeling in FME.