So I believe the pre-requisites are...
Finishing the Manor Quest.
Finding Rinny.
Modifying your own body.
And dated Rinny.
Milk and Pamper Bell, as you do it.
Then the option appears for Bell, to try the body modification in the succubi tower.
Then after that fails you can go to Bell, to talk about Pixie.
Also I'm not sure but I also did get the free fridge from Pixie.