Why can't third party controls be added to custom ribbon in Office PowerPoint via customUI.xml
<p>We are trying to put third-party add-in controls onto a custom ribbon via our add-in.</p>

<p>1) Customization via modifying the <strong>PowerPoint.officeUI</strong> is working:</p>

<pre><code>&lt;mso:customUI
    xmlns:x1="ThirdPartyAddin"
    xmlns:mso="http://schemas.microsoft.com/office/2009/07/customui"&gt;
    &lt;mso:ribbon&gt;
        &lt;mso:qat/&gt;
        &lt;mso:tabs&gt;
            &lt;mso:tab id="my_new_tab" label="my_new_tab"&gt;
                &lt;mso:group id="Test" label="New Group" autoScale="true"&gt;
                    &lt;mso:gallery idQ="x1:TheirFunction1" visible="true"/&gt;
                &lt;/mso:group&gt;
            &lt;/mso:tab&gt;
        &lt;/mso:tabs&gt;
    &lt;/mso:ribbon&gt;
&lt;/mso:customUI&gt;
</code></pre>

<p>We've extracted this xml by using File > Options > Ribbon Customization and inspecting the resulting PowerPoint.OfficeUI file.</p>

<p>2) Putting the below into our add-in code to be returned from  <strong>IRibbonExtensibility.GetCustomUI</strong> also referred to as RibbonXML:</p>

<pre><code>&lt;mso:customUI
    xmlns:x1="ThirdPartyAddin"
    xmlns:mso="http://schemas.microsoft.com/office/2009/07/customui"&gt;
    &lt;mso:ribbon&gt;
        &lt;mso:tabs&gt;
            &lt;mso:tab id="my_new_tab" label="my_new_tab"&gt;
                &lt;mso:group id="Test" label="New Group" autoScale="true"&gt;
                    &lt;mso:gallery idQ="x1:TheirFunction1" visible="true"/&gt;
                &lt;/mso:group&gt;
            &lt;/mso:tab&gt;
        &lt;/mso:tabs&gt;
    &lt;/mso:ribbon&gt;
&lt;/mso:customUI&gt;
</code></pre>

<h3>Notes:</h3>

<ul>
<li>With the mso:gallery In PowerPoint there are no UI XML error reports. </li>
<li>We removed  because the Quick Access Toolbar can't be modified that way.</li>
<li>Instead of the gallery shown here I also tried with a button which is listed as mso:control in PowerPoint.officeUI but which I had to change into mso:button for GetCustomUI otherwise PowerPoint would complain "0x80004005 Custom Controls cannot be specified as &lt; control >. Control type is required about ID x1:theirFunction2 </li>
</ul>

<h3>Problem</h3>

<p><strong>The item does appear but it's not working</strong>. it has the little arrow for gallery but has no icon. </p>

<h3>Observations</h3>

<ul>
<li>Ironically one can right click it and add it to the Quick Access Toolbar, where it's working again.</li>
<li><p>unloading the third party add-in while having the GetCustomUI add-in loaded will result in a minor update of the control</p>

<p><a href="https://i.stack.imgur.com/dAjDG.png" rel="noreferrer"><img src="https://i.stack.imgur.com/dAjDG.png" alt="see screenshot"></a></p></li>
</ul>

<h3>Related Documentation</h3>

<ul>
<li>From Cindy's comment I investigated "control" more appearantly there is a quite similar question, unfortunately the answer is unconfirmed: <a href="https://social.msdn.microsoft.com/Forums/en-US/d73d241c-64b8-4715-9ba7-aa95dbc0e24f/ctcontrolclone-ribbon-type-what-is-it-used-for?forum=os_binaryfile" rel="noreferrer">Can controls be clone</a></li>
<li>Also this is documentation on the "control" <a href="https://msdn.microsoft.com/en-us/library/dd953385(v=office.12).aspx" rel="noreferrer">MSDN</a></li>
</ul>