FWIW, I usually don't backsweeten until I'm literally ready to bottle; ie, in the bottling bucket...is there any reason you absolutely have to add this in the carboy?
Regardless, I suppose the answer would depend on how much room you have in the carboy...if there's room I'd try to dissolve whatever honey you want to add in a small amount of hot water, and then pour it in. Also, if you've racked it to the point you have no lees, you could perhaps use an aerating wand to stir the honey in to help w/ dissolution.