You can't just "add" it as an event for official consideration, there are only nine authorized events for the purposes of ranking in the Highland Games.

It could certainly be added as a challenge event, but I would suggest getting a bigger, heavier stone.
The problem I see would be with the dynamics of the event itself. Whenever doing a height event it takes forever for each competitor to attempt each height. Especially for a challenge event where no one is likely to know how high they need to come in and will likely come in early, thus making everything go longer.
That's the reason I don't really like the keg toss as a challenge event. It just takes too long and it doesn't look that dramatic. It doesn't hold the crowds interest that well when they watch.

My favorite challenge event would have to be the Anvil Toss (for distance) and the Idiot Stone (100 lbs, for distance). It goes quickly, it looks great for the audience because they can see this huge stone being hurled, and you don't have to stop and fix the bar like in the height events when someone hits it.