Are you talking about migrating client detail, or a mailbox on a server ?
If you are talking about a client, it should be basically as simple as copy/paste the Notes directories.
If you are talking about migrating a mailbox from one server to another, I think the easiest way is to create a replication stub on the destination server, then let the normal Replication task do its job. Once the mailbox has been properly set on the destination server, you can backup the original and remove it.
Your friendly Notes admin will know how to do that.
Pascal.