A very good question! I think just looked at the VAF10 guidance notes. They say :-
7.15 What is the main applicantâ€™s Certificate of Sponsorship number?
This is the number allocated to him/her by his/her employer/prospective employer. He/she is required to enter the number on their entry clearance application form and would not/will not have been issued entry clearance without doing so.
I am not impressed UKBA! Does anyone who designs the forms have more than half a brain? Didn't anyone appreciate that the Work Permit and employment visa might have been issued under the old regime, but the dependents visa is being issued under the new one? Come on, come on!
adnan_uk, as you might appreciate, under the new Tier 2 and Tier 5, if an employer wants to bring someone in from outside the EEA then they need to issue a Certificate of Sponsorship, which the individual would quote on their VAF9 form, and would thus be available for any dependent's VAF10 form.
I think the best you can do, as regards Q7.15, is to write something like "N/A, Work Permit issued prior to 27.11.08."
Hopefully UKBA will redesign the VAF10 form, or at least the guidance notes.