I don't think "easing your wife into this" is the right frame of mind. I'll try to get past my gut reaction of "ew" and look at this logically:
-Does she (your step daughter) still live with you? If so, NO. Do not start a relationship with someone who depends on you for food, housing, etc.
-What is her age? What is your age? Has she had serious relationships before or would this be her first? If she is young, or a lot younger than you, or inexperienced, I would still say NO. Might be ageism, and I know some people make a significant age gap work, but you are already starting from an untenable position.
-Do you have any reason to think your step daughter would be interested in you that way? Or is this just your fantasy? And have you thought about how your wife would feel having to schedule her time with you based on your time with HER OWN DAUGHTER??
Ultimately, though you may only be related by marriage, this does not sound like a good idea to me. Past the gut reaction "ew" factor (which by the way makes you, with this being your only post thus far, seem hella creepy), because I'm trying to give you a thoughtful answer, is the belief that having relationships with people of the same family (mother/daughter, father/son, brothers, sisters, even cousins) is NOT a good idea. Poly is already difficult enough to manage in terms of time and communication- why add a whole other layer of complication to it by having a family relationship involved too??
Pan Female, Hinge in a V between my mono (straight) husband, Monochrome and my poly (pan) partner, ThatGuyInBlack