You could modify pioneer3dx.xml, add a new link
like
<link name="simple_grey_box">
<inertial>
<mass value="0.01"/>
<origin xyz="0 0 0"/>
<inertia ixx="1" ixy="0" ixz="0" iyy="1" iyz="0" izz="1"/>
</inertial>
<visual name="base_visual">
<origin rpy="0 0 0" xyz="0 0 0"/>
<geometry name="top_geom">
<mesh filename="package://p3dx_description/meshes/top.stl"/>
</geometry>
<material name="TopBlack">
<color rgba="0.038 0.038 0.038 1.0"/>
</material>
</visual>
<collision>
<origin rpy="0 0 0" xyz="0.0 0 0"/>
<geometry name="pioneer_geom">
<box size="0.45 0.38 0.01"/>
</geometry>
</collision>
</link>
There also needs to be a joint
that makes this link
the child of an already existing one.
You could replace top.stl with a mesh of your own. Or make a box generated by urdf instead of loading a mesh. The gazebo urdf tutorials are the best place to start for understanding the urdf files: http://gazebosim.org/tutorials/?tut=r...