you extend the node, then you can add any fields you want.
For example:
<?xml version="1.0" encoding="utf-8" ?>
<component name="media_item" extends="ContentNode">
<interface>
<field id="thumbnail" type="string" />
<field id="content_type" type="string" />
<field id="title" type="string" />
<field id="entitlement" type="string" />
<field id="guid" type="string" />
<field id="duration" type="float" />
<field id="category" type="string" />
<field id="ratings" type="array" />
<field id="genres" type="array" />
<field id="daypart" type="string" />
<field id="program" type="string" />
<field id="episode" type="string" />
<field id="season" type="string" />
<field id="airdate" type="string" />
<field id="callsign" type="string" />
</interface>
</component>