You can use the "export-mailbox" and import-mailbox cmdlets. There is an option called "IncludeFolders" where you can select the "Junk e-mail" folder.
Further reading:
export-mailbox
http://technet.microsoft.com/en-us/library/aa998579(EXCHG.80).aspximport-mailbox
http://technet.microsoft.com/en-us/library/bb629586(EXCHG.80).aspxAnd, of course, you will need at lease E2k7 SP1 for these cmdlets.
Hope this gets you to the right track.