How do I install the OWASP ZAP certificate as a trusted system CA on Bluestacks?

I have a rooted Bluestacks Android. I would like to use ZAP to monitor its SSL traffic. I exported ZAP's SSL certificate as a .cer file

-----BEGIN CERTIFICATE-----
...
-----END CERTIFICATE-----

Now I wish to import this certificate into Bluestacks. The way I found was using an APK called "Root Certificate Manager". However, when I try to import the .cer file, I get an error response saying the ".cer" extension isn't supported. I guess I need to convert it but I can't find the right format to convert to.