This is what the missing mileage claim page has to say about your scenario:
Before making your request please note:
Your request must be made within 6 months of your original flight. For new members, only flights taken less than 1 month before the member's enrolment date will be accepted#.
#Club miles and Club sectors cannot be claimed for flights taken before enrolment in The Marco Polo Club.
Unless you can find a way to join MPC before your flights I think you might be out of luck.