No you can't add anything to the nectar. I suggest getting a yellow jacket trap and bating it with your old nectar solution when you change the nectar. You can also use a shallow bowl. Place the trap or bow away from the feeders. The yellow jackets or wasps with find it and go to it.

I'd just read to use red fingernail polish to coat yellow plastic flower portals because wasps are drawn to that color. So far, we haven't had wasps this year at our feeders.

I actually only have a problem with the yellow jackets trying to get the nectar at the feeders so that is why I use the yellow jacket traps. Bees and washps can't be bothered.

Penny