If you’re interested in giving it a try, take a look at this blog
entry ..this is one that
was the closest to matching reality that I could find:

http://www.eriugena.org/blog/?p=30

It relies on PGP (from PGP.com) to make the basic conversion…. you can
get a 30 trail license that does the trick, but PGP looks like it’s worth a closer look after the immediate task at hand is done… particularly since it appears to be the key to getting x509 cert keys into gpg.

In the post above, there is the assumption that PGP will strip out the
passphrase on the original keys that are on the thawte certs… well, it
doesn’t do that on the Mac version I acquired (I haven’t tried it from a
Microsoft world view)…
… it is just as well I suppose, since clobbering the private key passphrase in import seems like a security hole.

… ultimately, I had to skip that part of the blog and leave the
passphrase in place, find and compile an IDEA cipher (which PGP uses to
protect keys) as a plugin for GPG…

I finally found the idea.c source file (I’ll send it to you on
request), but I hae no guarantees as to its integity… the file I
downloaded was signed by a key that no longer shows up in public key
servers….. most of the links I found (like on the GnuPG site) were
broken… I sent a note the webminster about it, but back nothing so
far.

….after compiling and adding a line,

‘load-extension path-to-compiled-idea-code’

to the gpg.conf file, I was able to edit
the key and change the passphrase (since it could now understand how
to decrypt the old one)