Yes, I forgot to mention the ability to import .stl files into OpenSCAD. It works, but only if the .stl is 'well-behaved'. Often .stl's from other sources, and sometimes from OpenSCAD, have problems that make the import fail. Sometimes these can be fixed by running the .stl through a 'repair' program like Meshmixer. I've had mixed luck with this. If it comes in, you can usually subtract and add to it, scale it, etc.
As far as saving an OpenSCAD Customizer object, AFAIK it works like this: When you open a Customizer OpenSCAD file, the object is initially created with the default variable values that are written into the code. When you change those variable values with the Customizer tools, you are changing those values in memory only - they are not rewritten to the code of the file. You can save the current instance of your object as an .stl, but If you save the .scad file, and re-open it, it will revert to the original values written in the code.
However, there is some facility to save the Customized variables. At the top of the Customizer window there are some buttons to save, select, and reset 'presets'. I've fiddled with this only a little bit, and I think I described it a little in the instructions of one of my Configurable files. I think this can be used to save an object's customized variables into a file (I forget the file extension), and recover it. That could recover that customized object, but only back into the original OpenSCAD Customizer .scad file that created it originally. I don't think that could be a vehicle to pass objects between files.