Add flange / gripper to existing robot model (ur3)

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Hello everybody,

I am using the following model description of 6-DOF UR3 robot in this repo: urdf

I have read that it is not recommended to change the original xacro file (ur3.urdf.xacro). Now my goal is to add a simple cylindrical flange to the end of the kinematic chain by creating an own urdf.

flange.urdf.xacro looks like this:

<?xml version="1.0"?>

<!-- xacro flange for rg6 -->

<robot name="flange" xmlns:xacro="http://wiki.ros.org/xacro">
  <xacro:macro name="flange" params="prefix">

    <!-- flange joint and link -->
    <joint name="${prefix}flange_joint" type="fixed">
      <origin rpy="0 0 0" xyz="0 0 0"/>
      <parent link="${prefix}ee_link"/>
      <child link="${prefix}flange_link"/>
    </joint>

    <link name="${prefix}flange_link">
      <visual>
        <origin rpy="0 0 0" xyz="0 0 0.05"/>
        <geometry>
          <mesh filename="package://ur_description/meshes/ur3/visual/flange.stl" scale="0.001 0.001 0.001"/>
        </geometry>
        <material name="White">
          <color rgba="1.0 1.0 1.0 1.0"/>
        </material>
      </visual>
      <collision>
        <origin rpy="0 0 0" xyz="0 0 0.05"/>
        <geometry>
          <mesh filename="package://ur_description/meshes/ur3/visual/flange.stl" scale="0.001 0.001 0.001"/>
        </geometry>
      </collision>
      <inertial>
        <mass value="0.2"/>
        <inertia ixx="1" ixy="0.0" ixz="0.0" iyy="1" iyz="0.0" izz="1"/>
      </inertial>
    </link>

  </xacro:macro>
</robot>

I have included the corresponding joint for the extended move_group in ur3.srdf and added the tag to ur3_robot.urdf.xacro:

<xacro:include filename="$(find ur_description)/urdf/flange.urdf.xacro" />

Launching the config of ur3 in Rviz does not show the flange and brings up following messages in the terminal:

[ERROR] Joint 'flange_joint' declared as part of group state 'home' is not known to the URDF
[WARN] Skipping virtual joint 'fixed_base' because its child frame 'base_link' does not match the URDF frame 'world'
[INFO] No root/virtual joint specified in SRDF. Assuming fixed joint
[WARN] Kinematics solver doesn't support #attempts anymore, but only a timeout.

Can anybody give me a detailled instruction of how to handle this issue. I think my way of thinking is not correct at this point or something significant is missing. I have found Example: adding an end-effector to a robot, but it could not help me.