OpenSSL - unable to load pkcs7 object

First check your *.p7b file is simple or it is encoded by DER

Simply open that file in text editor, if open in readable form, then its simple, otherwise it is Encoded by DER)

If Simple then use this command:-

OpenSSL> pkcs7 -print_certs -in YourFileIn.p7b -out YourFileOut.cer

And if it is in Encoded by DER then use this command :-

OpenSSL> pkcs7 -inform der -in YourFileIn.p7b -out YourFileOut.cer

Convert *.pfx to *.key (pfx to key)

pkcs12 -in YourFileIn.pfx -nocerts -out YourFileOut.key

Convert *.p7b to *.pem (p7b to pem)

pkcs7 -inform der -in YourFileIn.p7b  -out YourFileOut.pem

Convert *.p7b to *.crt (p7b to crt)

pkcs7 -inform der -in YourFileIn.p7b  -out YourFileOut.crt

Convert *.p7b to *.cer (p7b to cer)

pkcs7 -inform der -in YourFileIn.p7b  -out YourFileOut.cer

For more help about conversion, Follow this Link