This is a simple example
<link name="dummy">
</link>
<link name="base_link">
<inertial>
<origin xyz="0.15 0.15 .2" rpy="0 0 0"/>
<mass value="10"/>
<inertia ixx="0" ixy="0" ixz="0" iyy="0" iyz="0" izz="0"/>
</inertial>
<visual>
<origin xyz="0 0 0" rpy="0 0 0"/>
<geometry>
<box size="0.01 0.01 0.01"/>
</geometry>
</visual>
<collision>
<origin xyz="0 0 0" rpy="0 0 0"/>
<geometry>
<box size="0.01 0.01 0.01"/>
</geometry>
</collision>
</link>
<joint name="dummy_joint" type="fixed">
<parent link="dummy"/>
<child link="base_link"/>
</joint>