Connecting any kind of additional geometry to a ROS robot model will typically not be done using any kind of GUI (there have been private projects, but I don't know of anything publicly available), but will require you to create something called a composite xacro.
For the models in ros-industrial/fanuc and ros-industrial/fanuc_experimental specifically (and essentially all robot support packages under ros-industrial), you could refer to Working with ROS-Industrial Robot Support Packages, specifically the Using the provided xacros section ("Example: adding an end-effector to a robot").
Do not edit any files in robot description or robot support packages directly. You should consider them read-only.
Note: while this approach is fairly generic, success depends on the exact structure of the files you are using. It will work for the ROS-Industrial robot support packages, but not necessarily for any other packages you may come across.
The principles would be exactly the same (ie: instantiate the two models and then add a joint which connects two links in two different models), but the specific names of the models, the links and the structure might need adapting.
