Are you importing the pkcs#7 file (as a reply from a CA) into an
existing private key entry? Or, are you simply trying to create a new
trusted certificate entry?

If I remember correctly, it should succeed in the first case but the
second case demands a *single* X.509 certificate in the file.

If you really want to create a trusted certificate entry, you can
first -printcert it using the -rfc option, choose the BASE64 format
certificate you need and copy/paste it into a file, and -importcert
it.

- Speedo

As mentioned before, since pkcs#7 is not a keystore provider; hence
the JDK keytool command will not be able to parse a pkcs#7 file.
However, you can easily extract the certificate chain
programmatically. See the method below:

private X509Certificate[] getCertificateChain(InputStream in) throws
CertificateException {
    CertificateFactory certFac = CertificateFactory.getInstance("X.509");
    CertPath certPath = certFac.generateCertPath(in, ENCODING);
    X509Certificate[] chain =
(X509Certificate[])certPath.getCertificates().toArray(new
X509Certificate[1]);
    return chain;
}