Sell it to someone with a V-twin for half price.
The zx6r and it's oil filter are designed to run on 40 weight oil.
Unless its going to be 100 degrees everyday, then it probably won't hurt much to just add a little.
But I wouldn't do a full change and replace all your oil with 50 weight.
20w50 is a recommended viscosity by Kawasaki from 32* to over 100*.
All you guys need to do is read the manual.
I run 20w50 in my bike and you'll be fine if your going to change it.
But since you asked if you can add it, I assume you have a different viscosity in your bike and I would not add 20w50 to whatever is in there.