Something like this sould work:
val = get(handles.listbox1,'Value'); %get selected item
str = get(handles.listbox1,'String'); %get content
set(handles.listbox2,'String',str{val}); %put item in 2nd listbox
or, to add the item to existing items in listbox2:
val = get(handles.listbox1,'Value'); %get selected item
str1 = get(handles.listbox1,'String'); %get content
str2 = get(handles.listbox2,'String')); %get content of listbox 2
set(handles.listbox2,'String',[str2 str1{val}]); %put item in 2nd listbox
