OS X 10.6 has OpenSSL with pkcs7, per the help text. Use man pkcs7 and man openssl for details of that.
With OpenSSL on OS X, the pkcs7 files are usually in .PEM or .DER format.
Per the man page "This PKCS#7 routines only understand PKCS#7 v 1.5 as specified in RFC2315 they cannot currently parse, for example, the new CMS as described in RFC2630."
OS X is using Secure Transport in more recent releases, and OpenSSL is around for legacy application use.
The "newer" infrastructure uses the security command, and that includes details such as "Possible formats are openssl, bsafe, pkcs7, pkcs8, pkcs12, x509, openssh1, openssh2, and pemseq. The default is pemseq if more than one item is being exported. The default is openssl if one key is being exported. The default is x509 if one certificate is being exported."
Whether these details address whatever you're interested in here, or what sort of problem you're encountering here, I don't know. I'm guessing you're looking to use these pksc7 certificates in some fashion? Single-signon or s/mime would be typical for pkcs7, but details can depend on the requirements and the particular client or server in use.