Connecting AD users to 5.5 mailbox.

I am currently running Exch 5.5 on NT 4 and in the process of migrating to Exch 2000 on W2k. I have installed ADC and Exch2k on the new mail server. ADC was only able to connect about 1/2 the users to their mailbox on 5.5. I need to know how to manually connect the remaining AD users to their mailbox on 5.5 so I can then move their mailbox to the new E2k server.