Texturize robot consisting of STL-files

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I created robots links with CATIA exported them as STL an constructed my own robot with URDF. Now I'm able to move my little mobile robot around the world in Gazebo. Yeah. Then I mounted an Universal Robot Arm on top of my rover and there where textures on the arm. I looked in the XACRO-Description and found out, they use DAE files with textures to colorize the robots links. Is it possible (and if with which software) to import STL-files attach textures on it and export it as DAE as easy as possible? How would you texturize my existing robot with STL files?

Thanks for your help Max