as seerah said, what your asking is doing randoms of your choice, wich ofc would totally collapse the idea of using the random dungeon finder.
If you dislike healing some instances, then use the specific hc bit instead of random, you can choose what you want to do then
Besides, everyone in the game sooner or later does an instance they hate/dislike, but queing for randoms and then choose wich not to go and wich you wanna go, please dont because a lot of people get annoyed from you if you leave the second you enter, and it gives you a bad reputation.
wich eventually will result in people kicking you the moment they can.