Não tem segredo, se você manjar só um pouco de XML é só fuçar no arquivo que fica no diretório de algum tema e modificar até chegar onde você quer...
um bom exemplo é o arquivo do meu tema pois ele está com a seções separadas...
baixa ele daqui e modifica, na dúvida pergunta se eu puder ajudar estamos aí
http://gnomosapiens.wordpress.com/2006/06/29/rude-buay-gdm-theme/o arquivo XML dele.
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE greeter SYSTEM "greeter.dtd">
<greeter>
<!-- background -->
<item type="pixmap">
<normal file="background.png"/>
<pos x="0" y="0" width="100%" height="100%"/>
</item>
<!-- bottom bar -->
<item type="rect">
<normal color="#000000"/>
<pos y="100%" x="0" width="100%" height="42" anchor="sw"/>
<box xpadding="10" spacing="10" orientation="horizontal">
<!--session -->
<item type="rect" id="session_button" button="true">
<pos y="50%" width="box" height="box" anchor="w"/>
<box xpadding="0" spacing="2" orientation="horizontal">
<item type="pixmap">
<normal file="icon-session.png"/>
</item>
<item type="label">
<normal font="Bitstream Vera Sans Bold 12" color="#ffffff"/>
<prelight font="Bitstream Vera Sans Bold 12" color="#ff9c36"/>
<active font="Bitstream Vera Sans Bold 12" color="#dc292b"/>
<pos y="50%" anchor="w"/>
<stock type="session"/>
</item>
</box>
</item>
<!--language -->
<item type="rect" id="language_button" button="true">
<pos y="50%" width="box" height="box" anchor="w"/>
<box xpadding="0" spacing="2" orientation="horizontal">
<item type="pixmap">
<normal file="icon-language.png"/>
</item>
<item type="label">
<normal font="Bitstream Vera Sans Bold 12" color="#ffffff"/>
<prelight font="Bitstream Vera Sans Bold 12" color="#f2562d"/>
<active font="Bitstream Vera Sans Bold 12" color="#dc292b"/>
<pos y="50%" anchor="w"/>
<stock type="language"/>
</item>
</box>
</item>
<!-- reboot -->
<item type="rect" id="reboot_button" button="true">
<pos y="50%" width="box" height="box" anchor="w"/>
<box xpadding="0" spacing="2" orientation="horizontal">
<item type="pixmap">
<normal file="icon-reboot.png"/>
</item>
<item type="label">
<normal font="Bitstream Vera Sans Bold 12" color="#ffffff"/>
<prelight font="Bitstream Vera Sans Bold 12" color="#ff9c36"/>
<active font="Bitstream Vera Sans Bold 12" color="#dc292b"/>
<pos y="50%" anchor="w"/>
<stock type="reboot"/>
</item>
</box>
</item>
<!-- halt -->
<item type="rect" id="halt_button" button="true">
<pos y="50%" width="box" height="box" anchor="w"/>
<box xpadding="0" spacing="2" orientation="horizontal">
<item type="pixmap">
<normal file="icon-shutdown.png"/>
</item>
<item type="label">
<normal font="Bitstream Vera Sans Bold 12" color="#ffffff"/>
<prelight font="Bitstream Vera Sans Bold 12" color="#ff9c36"/>
<active font="Bitstream Vera Sans Bold 12" color="#dc292b"/>
<pos y="50%" anchor="w"/>
<stock type="halt"/>
</item>
</box>
</item>
<!-- quit -->
<item type="rect" id="disconnect_button" button="true">
<normal/>
<show modes="flexi,remote"/>
<pos y="50%" width="box" height="box" anchor="w"/>
<box xpadding="0" spacing="2" orientation="horizontal">
<item type="pixmap">
<normal file="icon-quit.png"/>
</item>
<item type="label">
<normal font="Bitstream Vera Sans Bold 12" color="#ffffff"/>
<prelight font="Bitstream Vera Sans Bold 12" color="#f2562d"/>
<active font="Bitstream Vera Sans Bold 12" color="#dc292b"/>
<pos y="50%" anchor="w"/>
<stock type="disconnect"/>
<show modes="remote"/>
</item>
<item type="label">
<normal font="Bitstream Vera Sans Bold 12" color="#ffffff"/>
<prelight font="Bitstream Vera Sans Bold 12" color="#f2562d"/>
<active font="Bitstream Vera Sans Bold 12" color="#dc292b"/>
<pos y="50%" anchor="w"/>
<stock type="quit"/>
<show modes="flexi"/>
</item>
</box>
</item>
</box>
</item>
<!-- hostname and clock -->
<item type="rect">
<pos x="100%" y="100%" width="box" height="42" anchor="se"/>
<box xpadding="10" spacing="10" orientation="horizontal">
<item type="label">
<pos x="100%" y="50%" anchor="e"/>
<normal font="Bitstream Vera Sans Bold 12" color="#ffffff"/>
<text>%h //</text>
</item>
<item type="label" id="clock">
<pos x="100%" y="50%" anchor="e"/>
<normal font="Bitstream Vera Sans Bold Italic 10" color="#ffffff"/>
<text>%c</text>
</item>
</box>
</item>
<!-- password box -->
<item type="rect">
<pos x="50%" y="50%" width="box" height="box" anchor="c"/>
<box xpadding="0" ypadding="0" spacing="5" orientation="vertical">
<item type="rect">
<pos x="0" y="0" width="box" height="box" expand="true"/>
<normal color="#ffffff" alpha="0.3"/>
<box xpadding="50" ypadding="15" spacing="10" orientation="vertical">
<item type="label" id="pam-prompt">
<pos x="0"/>
<normal font="Bitstream Vera Sans Bold 14" color="#000000"/>
<stock type="username-label"/>
</item>
<item type="rect">
<normal color="#000000"/>
<pos width="190" height="24"/>
<fixed>
<item type="entry" id="user-pw-entry">
<pos y="1" x="1" width="-2" height="-2" anchor="nw"/>
</item>
</fixed>
</item>
<!-- timer warning -->
<item type="label" id="timed-label">
<show type="timed"/>
<normal font="Bitstream Vera Sans Bold 12" color="#000000"/>
<stock type="timed-label"/>
</item>
</box>
</item>
</box>
</item>
<!-- caps lock warning -->
<item type="label" id="caps-lock-warning">
<pos x="-6" y="-48" anchor="se"/>
<normal font="Bitstream Vera Sans Bold 12" color="#dc292b"/>
<stock type="caps-lock-warning"/>
</item>
<!-- pam error -->
<item type="label" id="pam-error">
<pos x="6" y="-48" anchor="sw" />
<normal font="Bitstream Vera Sans Bold 12" color="#dc292b"/>
<text></text>
</item>
</greeter>