Use the model editor - find the entity you want to change and open it. Make sure the object has a body defined. Make sure the body mass is set to a non-zero value.
Then go to settings->User Defined Vars and change the type to "Object" - pick either grab or push as the sub-type. Enter the body's name into the "Main Physics Body" feild. The object will now be able to be moved around.
This answer is just a quick list of things you need to do. If you haven't used the model editor yet - I advise you check out:
http://wiki.frictionalgames.com/hpl2/tut...tutorial_1