I have successfully refilled a Mathmos Glitterball Lava Lamp with pure 100% (not diluted) Glycerin.
It works perfectly.
I loosened the cap, gently poured off the existing liquid and refilled with pure Glycerin leaving enough room for expansion at the top.
My cloudy Gliiterball that was two inches low is now perfect in every aspect.
If you are in the same boat as me and do not want to risk what is left of your Glitterball, put your mind to rest and forget about those hesitations, it worked without a hitch!
Hope this is helpful.