Let me put some points clear for you:

1) Equivalence of O and A level are compulsory at the time of CSS application.
2) The General Certificates of Education by CAIE are not acceptable or required by the Commission.
3) Your equivalence certificate holds the legitimate date of your qualification.
4) The equivalence certificate holds your accurate to two decimal places result percent and marks.
5) In theory, you can forge this information in the form by putting a calculated date and calculated marks.
6) However as in Point 1, This would imply that you already have your equivalence certificate at the time of application for CSS.
7) No information that is provided to the commission can be changed as per the rules 2019.
8) You would have to be very influencial to have the O level equivalence generated on back-date.
9) Eventually you would be caught and you a Police case would be registered for forgery.

10) If I were you, I would go by the safe route and abide by the due process; appear for the Olevel exam, have the equivalence certificate and then appear in CSS the next year. it's no big deal if you weigh in the potential risks as mentioned in point 9.

I hope you get the idea. Besides, you can talk to anybody else on the matter as well. But as a word of caution, if you cannot pull off point 8 then don't risk it.