What is it about the beer that you are not happy with? How long was it lagered? The only reason to add yeast to a lager at bottling is if it's been lagering for a long time. The longer it lagers, the more time for yeast to settle out, but this would take a while. I lager most of mine for 4 weeks and have never added yeast. They have always carbonated just fine. I imagine if I ever brewed a Marzen in March and didn't bottle until August, I would probably add some yeast.
As for too little yeast for carbonation producing off-flavors?... I've never heard that discussed before. But perhaps with carbonation, since the amount of fermentable sugar is so small, you either have enough yeast or you don't.
If I were to add yeast to a lager for carbonation, I would ideally use a dry lager strain. You wouldn't need much... maybe half a pack per 5 gallons. I guess in a pinch I would use a clean dry ale strain, but I would be worried about flavor a bit with such a different yeast.