My niece did that too!! Her mom eventually bit the bullet and put away the pull-ups for good. They both had to clean out underpants for about a week, dunking the panties in the potty to reinforce where poop was supposed to go, but then the miracle lightbulb went off in her head and she finally started pooping in the potty.
I've been training my 3 year old son this summer, and what helped him (and his big sister, and my neighbor, and my sister's kids:) ) is a children's book called "Everyone Poops" by Taro Gomi. It is a little graphic, but it really helped all of these kids with the lightbulb moment, realizing that poop goes in the potty. We would sit in the bathroom around the time of day I knew he usually had a BM, with him on the potty, and read the book 2 or 3 times during a session. Then we'd take a break and do something different for 1/2 hour and then try again. Eventually he put 2 & 2 together.
Good luck to you!