Good News: If you use your Hydro Floss properly, you should't need to put anything extra in it.
There is nothing wrong with using your essential oils separately either, so you don't have to throw them away. Just don't put them in you Hydro Floss.
Here are some items you CAN put in your HydroFloss safely:
PerioTherapy can be used in both your HydroFloss as well as a stand alone mouthwash. You only need a tiny bit in your HydroFloss though. Perhaps two thimbles worth. (A small amount)
However, to be perfectly honest with you. If you use the HydroFloss properly, you will probably find that you don't need to add other things.
I also know that some people like to do so anyway. Just don't use anything sticky, oily or gritty. If it is just a liquid and isn't sticky, oily or gritty, you should be fine.