Unfortunately, the policy is very clear. You would need to present a very compelling case as to why making a claim wasn't possible within 6 months, but even then the answer would likely be no.

Allowing retro claims is a goodwill gesture for new members but of course there has to be a cut off. Some other airlines (eg. BA) only allow claims for flights taken in the three months before joining their FF scheme, so this policy is already quite generous.

You might try sending a message via the feedback link below but unless you can convince them why you are a special case, I suspect those points are gone.

https://www.qatarairways.com/en/help.html#feedback