Simply use vboxmanage.exe utility.

C:\Program Files\Oracle\VirtualBox>VBoxManage.exe registervm “E:\Machines\7 Lite\7 Lite.xml”
Oracle VM VirtualBox Command Line Management Interface Version 3.2.4
(C) 2005-2010 Oracle Corporation
All rights reserved.

Then 7 Lite will appear in your VirtualBox GUI.