I had to follow the same steps for the same reason: Very heavy IT security and no means to manually install.
Arduino IDE: 1.8.1
ESP8266 Board Manager: 2.3.0
Installation Method: Normal (not portable)
Clean install on corporate machine. Install ESP8266 Board Manager. Failed: Could not download zip, SSL error.
Clean install on non-corporate machine. Install ESP8266 Board Manager. Success.
Copy \Users\xxxx\AppData\Local\Arduino15 to usb stick on non-corporate machine
Insert usb stick on corporate machine and do a virus scan just to be sure.
Copy Arduino15 to \Users\xxxx\AppData\Local\Arduino15
Start it up. There was a minor one-time warning. All is good.
Note that parts of the file path are hidden so you have to type it in rather than navigate folder-by-folder.