Wirelessly posted (BlackBerry Curve 8900)
On your Curve 8300 go to Options > Advanced Options > Enterprise Activation. Scroll down so your cursor is after the activation date. Hold down ALT and type CNFG. Go to the Wireless Sync option and set it to No, then save the changes. You should now be able to use the Desktop Manager to backup the Messenger contacts.
I also do believe that once you activate the 8900 on the BES that those would be copied back, but I could be wrong.