lbs and kg are already possible (simply click on the lightblue colo(u)red unit appreviation). Stone should not be that difficult if somebody can tell met what 2 or 3 letter abbreviation to use and what conversion rate to use compared to KG. 

yeah I use lb and kg now on EV and it's very handy which is why it's a shame on the occasions when passenger details are passed to me in stone and I have to grab a calculator to work it out (14lb in a st)

To 5 DP there are 6.35029 kg in a stone

I would say that "ST" would be appropriate as I believe that they don't use the measurement in USA - just the UK until we can finally decimalise everyone!

This should be 20 minutes max to implement. I'll try to not forget upcoming days. Feel free to remind me subttle or less subttle 😉