Converting .cer to .pem?
Actually, the .cer and .pem extensions are quite confusing for me. From the command output provided, I think your cas.cer should be bas64 encoded, which is the format accepted by OpenSSL without any additional parameter.
So, you may try to copy the cas.cer to cas.pem (no conversion is needed, just change the filename).
I've used this site in the past to convert a cert with good results.
https://www.sslshopper.com/ssl-converter.html