I have some sample code:
    
When it first loads I can't select the disabled item to transfer.
I transfer an item from RadListBox1 to RadListBox2 then use transfer arrows to move that item back.
The disabled item is now selected and I can use arrows to move the disabled item to RadListBox1..... if that makes sense.
                                | <telerik:RadListBox ID="RadListBox1" runat="server" AllowTransfer="True" TransferToID="RadListBox2"> | 
| <Items> | 
| <telerik:RadListBoxItem Text="Item 1" /> | 
| <telerik:RadListBoxItem Text="Item 2" /> | 
| <telerik:RadListBoxItem Text="Item 3" /> | 
| <telerik:RadListBoxItem Text="Item 4" /> | 
| <telerik:RadListBoxItem Text="Item 5" /> | 
| </Items> | 
| </telerik:RadListBox> | 
| <telerik:RadListBox ID="RadListBox2" runat="server"> | 
| <Items> | 
| <telerik:RadListBoxItem Text="Disabled Item" Enabled="false" /> | 
| </Items> | 
| </telerik:RadListBox> | 
I transfer an item from RadListBox1 to RadListBox2 then use transfer arrows to move that item back.
The disabled item is now selected and I can use arrows to move the disabled item to RadListBox1..... if that makes sense.
