I daresay you could but at that temperature a lot will disappear. If soft enough you could incorporate some after it has cooled. Personally I wouldn't bother. Or is this instead of sugar syrup? If so, the amounts will be exactly the same per kg of sugar as the liquid feed versions.
Interesting question that doesn't appear to have been asked before, but probably because it seems a bit obvious . . .
As RAB said, a lot would evaporate at the temperatures involved. The kitchen would likely be the recipient of the all pervasive aroma and could cause severe severe earache . . . from SWMBO, or even worse.
Being hygroscopic, the fondant will absorb water, but not necessarily a tincture of thymol. As the fondant cools, it might be possible to mix the thymol tincture into the fondant, but getting anything like a satisfactory distribution would take some doing and the odds of achieving success are remote.
Either way, it sounds like something that doesn't want to be done in a house.
Can anyone make any sence of this for me, and would this mean i can add it to the fondant as it says it dissolves with glycerin and don't i add glycerin to the sugar mix to make it stay soft ish?
Thymol also dissolves in 120 parts of glycerin. In making solutions of thymol with glycerin and water, it is best to triturate the thymol in a mortar with a small portion of alcohol, until it is dissolved, then add the desired menstruum. Thymol boils at 230° C. (446° F.) (Doveri and Lallemand).