You could probably copy and paste the entire common.js to your common.js, but you would have to change the links like 'MediaWiki:Common.css/calc.css' to 'u:runescape:Common.css/calc.css'. To do that, you can press Ctrl + H in the code editor to find and replace iterations of 'MediaWiki' to 'u:runescape'.
The RuneScape Wiki appears to use a custom script to import all of their other scripts. Even if you did copy the entire Common.js, I don't think it would work as they are directly using AJAX requests which do not recognize the the "u:domain" format for interwiki imports. Plus, you would be importing way more than just the calculator. I suggest the following.
I had a friend make a Calculator for me. They told me to make the calculator.js read-only to prevent people from editing it, but I don't see a read-only option on the .js page. Is the .js page automatically protected?