Question updated
I currently add plugins to my cordova project using the command cordova plugin add x. I believe this uses npm in the background.
Does anyone have any idea how I can switch npm out for yarn js (within cordova, I want cordova to use yarn instead of npm), since it would make such a massive difference in terms of caching (other than updating the cordova cli myself).
Conclusion
Dec 2016
It seems that installing yarn means that yarn takes over from npm as the default package manager, and thus things are speeded up by default and that no tinkering with Cordova is required.
Sep 2017
It seems that I was mistaken, apparently, Cordova is hardcoded to use npm, https://issues.apache.org/jira/browse/CB-12242. I will re-open this issue to request Apache to make this an option.
It seems the short answer is no - it is not currently possible, but I am going to request an update from Apache: https://issues.apache.org/jira/browse/CB-12242