Possible to populate a subform from a stateless form?

Possible to populate a subform from a stateless form?

Hi Zoho,
is it possible to send data from a stateless form to a subform in a 'real' form? The annotated screenshot below describes what I mean.


Background
I am creating a multi-stage registration process. Currently I've got the form set up using checkboxes to provide a 'wizard' effect:

However, my client doesn't like the user experience of this (saying rightly that it's not very intuitive). He wants me to come up with a stateless form approach with "Next" and "Back" buttons down the bottom.