M.A.
I don't think that's an awful habit; I agree with the others - don't force the issue. Let her develop this skill in her own time. She will hold it and you don't want to end up in the ER with impacted poop. She's used to standing and pooping and now you want her to sit and she's just not comfortable yet.
My ped suggested we do this - buy pull-ups and put them where my daughter can reach them. When she has to poop, she takes off her panties and puts on a pull-up and hides and goes. It gives her a little independence, but she doesn't poop in her panties. It's worked so far to alleviate the diaper battles we were having because she still feels like a big girl, not wearing 'diapers'.
good luck!