New .gnupg directory: Import old secret keys into new install

Yes, the OpenPGP "secret key" and "secret subkey" packets contain both public and private parameters. You can verify this by using pgpdump to examine the exported key:

$ gpg --export-secret-key grawity | pgpdump
Old: Secret Key Packet(tag 5)(1854 bytes)
    Ver 4 - new
    Public key creation time - Sat Oct 31 14:54:03 EET 2009
    Pub alg - RSA Encrypt or Sign(pub 1)
    RSA n(4096 bits) - ...
    RSA e(17 bits) - ...
    Sym alg - CAST5(sym 3)
    Iterated and salted string-to-key(s2k 3):
        Hash alg - SHA1(hash 2)
        Salt - 12 24 0f e1 5b 7e e2 46 
        Count - 65536(coded count 96)
    IV - 91 a3 44 71 47 87 a4 ba 
    Encrypted RSA d
    Encrypted RSA p
    Encrypted RSA q
    Encrypted RSA u
    Encrypted SHA1 hash

This is true for most asymmetric key systems, not just OpenPGP.

I think you are using the wrong program; pgpsm is used to sign, check, encrypt or decrypt using the S/MIME protocol.

I do not have pgp4win at hand, but according to the man page, this should export your public and secret key:

gpg -o XXXXXXX_private.p12 --export [key id] --export-format pkcs12 --cert

The info on pkcs12 is the following,

pkcs12 Only binary blocks are output; the default file extension is .p12; a signed key must be paired; and input must match exactly one key. In this case, --cert is required.

so I included the --cert option, without reflection about that option:

--cert This option is the X.509 issuer long name or the 32-bit or 64-bit key ID, if the signing key is available.

I did some more tests (now with gpg4win), and partially have to contradict myself. The gpgsm tool in gpg4win describes itself as

gpgsm is a tool similar to gpg to provide digital encryption and sign- ing services on X.509 certificates and the CMS protocol. It is mainly used as a backend for S/MIME mail processing.

which indeed sounds correct.

So, your command posted in the question seems totally sensible, I only have two more clues:

• You can try to use a ASCII armored output via the -a option
• And there is an option concerning the charset of the exported key, which often is a problem with (especially older) windows programs:

--p12-charset name gpgsm uses the UTF-8 encoding when encoding passphrases for PKCS#12 files. This option may be used to force the passphrase to be encoded in the specified encoding name. This is useful if the application used to import the key uses a different encoding and thus will not be able to import a file generated by gpgsm. Commonly used values for name are Latin1 and CP850. Note that gpgsm itself automagically imports any file with a passphrase encoded to the most commonly used encodings.