config.xml

<!-- /app/code/local/MyCompany/MyModule/etc/config.xml -->

<?xml version="1.0"?>
<config>

  <adminhtml>
    <layout>
      <updates>
        <mycompany_mymodule>
          <file>mycompany/mytemplate.xml</file>
        </mycompany_mymodule>
      </updates>
    </layout>
  </adminhtml>
</config>

mytemplate.xml

<!-- /app/design/adminhtml/default/default/layout/mycompany/mytemplate.xml -->

<?xml version="1.0"?>
<layout>
  <adminhtml_customer_index>
    <reference name="content">
      <block type="block/typer_here"/>
    </reference>
  </adminhtml_customer_index>
</layout>