Steps to reproduce: 
Insert a SELECT element containing some OPTION, select it and open InsertSelect dialog.

Expected: The SELECT to be loaded to the dialog and edited after closing it.
Actual: The SELECT is not loaded and a new SELECT has been inserted when closing the dialog.

Workaround: 
<telerik:RadEditor ID="RadEditor1" runat="server" RenderMode="Lightweight">
    <Tools>
        <telerik:EditorToolGroup>
            <telerik:EditorTool Name="InsertFormSelect"></telerik:EditorTool>
        </telerik:EditorToolGroup>
    </Tools>
    <Content>
		<p>Test</p>
    </Content>
</telerik:RadEditor>

<script type="text/javascript">
	var editorPrototype = Telerik.Web.UI.RadEditor.prototype;
	var getSelectedElement = editorPrototype.getSelectedElement;
	editorPrototype.getSelectedElement = function (range) {
		var selected = getSelectedElement.call(this, range);
		return selected && selected.nodeName == "OPTION" ? selected.parentNode : selected;
	};
</script>