Transferring a private key

I have never used this software so please excuse my ignorance. I installed the package on a test machine and transferred some files from a client. All tests were successful. I now need to implement the solution for my user community, specifically one user who will be dealing with this client. How can I transfer the private key from the test machine to the user machine? I would prefer not to have to generate a new private and public key because the client has already started transfering files with the existing public key that is tied to this private key.

Am I making sense ???

If both instances are GnuPG, you could
simply make an archive out of the ~/.gnupg
directory and extract it on the other system into
the correct users directory.

If you use another OpenPGP product,
you must look there for a general export
of the keys.

I tried this and unfortunately it did not work. I think I need to find an alternate solution. I simply cannot allow our entire company to rely upon a single PC for this function. Do you know of any other products that allow the sharing of the private key?? I am the Enterprise Administrator of the Domain and CIO of the company. I see no security risk in sharing the private key between approved users.