To make sure you don't hurt the grains, after straining them out of freshly made kefir, you can add in sugar/sweetener to taste and about 1 tsp of vanilla extract to the fresh kefir milk.
Most store bought brands also add in a bit of fresh milk to 'lessen' the tang of the kefir, but that's optional.
If you add sugar to the milk while the grains are 'doing their thing' then it makes the yeast component 'fatten up' and might give it more 'bubbles' and a tarter taste (like sourdough). Vanilla extract should be safe enough, if it's not made witih tons of preservatives, but the alcohol in it might affect the grains. I would add vanilla extract AFTER brewing and straining out the grains, just to be safe.
I'm sorry I missed this post earlier, I've been away from the computer for most of the week. /hug