Adding search criteria to admins Email Manager

[woostar]

I have added an additional field to the 'user' table called 'geoip'. and have added this field to the admin user panel using hook 'useradmin_edit_column1'
E.G:

<plugin active="1" executionorder="5">
<title><![CDATA[[GeoIP] - Admin Edit]]></title>
<hookname>useradmin_edit_column1</hookname>
<phpcode><![CDATA[print_table_header($vbphrase['geoip']);
print_input_row($vbphrase['geoip'], 'user[geoip]', $user['geoip'], 0);]]></phpcode>
</plugin>

How do I add to admins Email Manager so I can send out emails by users matching this field (and is it as easy as finding the right hook)?

Thanks in advance