Thanks for your help. I think i should do in the below steps.
1. point his domain name to our DNS server.
2. use whm->copy an account from another server without that account's password(this one looks easier than "Copy an account from another server"^-^). when i click "copy an account from another server without that account's password", i get the below information.
Server to copy from (ip or FQDN): I will input ip of the server that the account resides on now.
Domain Name (only required if web root protection is enabled on the remote server):should i input our client's domain name here?i do not know if web root protection is enabled on the remote server.
Username to copy: I will ask our client.
User's password:I will ask our client.
Give new account an ip address: I think now it is not nessesary, i will uncheck this box.
The Remote Server must be cPanel3 or later. If the server that the account resides on now does not use cpanel, then i cannot use this function to transfer his account, right?
Another question, if i use "copy an account from another server without that account's password", could it also copy all the emails to our host server?