Yes you can. this example has 3 levels of menus
<customUI xmlns="http://schemas.microsoft.com/office/2006/01/customui" >
<ribbon >
<tabs >
<tab
id="Tab1"
label="Tab1">
<group
id="Group1"
label="Group1">
<menu
id="Menu1"
imageMso="FormulaMoreFunctionsMenu"
label="Menu1"
size="large">
<menu
id="Menu2"
label="Menu2">
<button
id="Button1"
label="Button1"/>
<button
id="Button2"
label="Button2"/>
</menu >
<menu
id="Menu3"
label="Menu3">
<button
id="Button3"
label="Button3"/>
<button
id="Button4"
label="Button4"/>
</menu >
<menu
id="Menu4"
label="Menu4">
<button
id="Button5"
label="Button5"/>
<menu
id="Menu5"
label="Menu5">
<button
id="Button6"
label="Button6"/>
<button
id="Button7"
label="Button7"/>
</menu >
</menu >
</menu >
</group >
</tab >
</tabs >
</ribbon >
</customUI >