How can I make a geomety more complex

edit

Hi I want to add my Pinoneer-3at model. In this case I have to change the geometry of the "top-plate". The geometry should be 2 boxes. How can I solve it?

This doesn't work. In Gazebo I see only the first one

<link name="top_plate">
  <inertial>
   <mass value="0.1"/>
   <origin xyz="-0.025 0 -0.223"/>
   <inertia ixx="1.0" ixy="0.0" ixz="0.0" 
               iyy="1.0" iyz="0.0" izz="1.0"/>
   </inertial>
   <visual name="base_visual">
      <origin xyz="0 0 0" rpy="0 0 0"/>
      <geometry name="pioneer_geom">
         <box size= "0.20 0.50 0.01" />
      </geometry>
      <geometry name="pioneer_geom2">
          <box size= "0.60 0.20 0.01" />
       </geometry>
       <material name="TopBlack">
          <color rgba="0.038 0.038 0.038 1.0"/>
       </material>
   </visual>
   <collision>
       <origin xyz="0 0 0" rpy="0 0 0"/>
       <geometry>
          <box size="0 0 0"/>
       </geometry>
   </collision>
</link>