Yeast question.

I have already kegged my Old Ale. I know it was under attenuated it came out at 10% but I was shooting for about 13.5-14% I used London Dry Ale yeast. it quit at 10% I have already kegged this beer thinking that I thought I would like it a bit sweet. However I am now thinking it is too sweet. Can I pitch some yeast directly in the keg in the beer as it is? maybe a champagne yeast? OR would that dry it out too much? I would like a little sweetness in there.
Am I stuck with what I've got?

If you're going to try that, I would make a starter of WLP099 Super High Gravity yeast and pitch that. The Champagne yeast isn't going to do anything for you unless there are some simple sugars left.