In theory, you should be able to credit them to QR:
"If you've joined Privilege Club after your trip, make sure to submit your claim request within 90 days of your trip."
https://www.qatarairways.com/en/Priv...ing-avios.html

However, I am still waiting for my retro-credit requested in May for an Explorer Pass flight in G Class.

Edit: Nevermind. Domestic flights in G aren't eligible as per https://www.qatarairways.com/en-qa/P...NAIRLINES.html