After the allocation day can I then request to be in the waiting list to a completely new school for which my DD did qualify but was not in our earlier 6 choices in the CAF form?
I am not sure why you want to do that ?
You list 6 schools on the CAF then you go on the waiting list for a 7th school , that suggests that your prefrence is not correct. The CAF system is designed such that you will be allocated a places which takes your preference into account.
Putting school B in to 2nd does not prejudice your chance of getting into school A which may be your 1st choice. The schools do not know your preference when they receive your application via the CAF system as such they assign places based on the admission criteria only.
There is an excellent diagram on this site which shows how the system works.viewtopic.php?f=30&t=28833
As far as I understand, you can contact the 7th school and ask to go on the list.