Add the following toolBar configuration element to your custom .xxe file:
<toolBar> <insert /> </toolBar>
After the insert element, add one or more separator and/or button elements. Example:
<toolBar>
<insert />
<separator />
<button toolTip="TEST" icon="xxe-config:common/icons2/help.gif">
<command name="alert" parameter="TEST"/>
</button>
</toolBar>