Add more fields in payment process

Hi All,
I know that it is not possible to change checkout form in Wix Store but a customer asked me to add a Birthday and Gender field somewhere along the process because it is necessary for her to know those details in order to provide the right product.
Any ideas of how to implement it?
I thought maybe it will be possible to catch the check out event and add my own form in the middle.
I’ll appreciate your thought
ROnen