I'd just pour the sugar in.
In addition to what others have said, if the beer has hit high krausen (i.e., big old cap of foam on top and airlock bubbling like mad) the ABV in your beer will have probably gotten up to the point that the alcohol will also help to kill many of the bugs on the sugar.
While the sugar won't dissolve immediately, it will dissolve over a period of a couple of weeks.
I can't say exactly how it will affect your ferment, however. The sudden influx of sugar might restart fermentation, or it might shock the yeast and give you attenuation problems. In the latter case, you might need to pitch more yeast or rouse the existing yeast.
There's also the option of Not Messing With It. Your tripel might come in a little low in alcohol and a bit chewy for the style, but so what? As long as it tastes good, it is good.