right click while having walls selected, choose roof/floor line, you have several options, top of wall, bottom of wall and edit in place. If you choose roof, then to Automatically have it extend, choose A and then select the roof items. It should extend them up.
This is using items that I know were in the 08 release, I am not positive if 07 had this option at this specific location though.
This does work through Xrefs