How to add external js to magento

Add to your layout:

<reference name="head">
   <block type="core/text" name="my.external.js">
      <action method="setText">
        <text>
           <![CDATA[<script type="text/javascript" src="https://domain.com/my_file.js"></script>]]>
        </text>
      </action>
   </block>
</reference>