Programming a plug-in able to import IFC files would be a good start.
First, you should learn how to write a plug-in, then create objects of the com.eteks.sweethome3d.model package from the data read in an IFC file and add these objects to the edited Home instance. The magic of MVC will make them appear at screen automatically.
As far as I know, IFC / BIM files exists at binary and XML formats, and it should be more easy to decode the XML files using Java DOM API (or SAX API if the file is big).