You have just became a reseller.
You have some contacts that want to move to your service.
You set up their accounts in WHM and give them their welcome emails with usernames, passwords, etc.
You'll need to help them set up their email accounts again, in Mail Manager in the CPanel.
They will have a default mail account that is also their username.
They can then access the mail (after DNS propagates) to:
If they are using their default account, they only have to log in with
Log in: username
If they are using an account they or you helped create, they must log in with
Log in: firstname.lastname@example.org
There is not a way to transfer their old emails into webmail, unless they forward them from somewhere else, to their webmail account.
Transferring emails will be a task that is up to you to accomplish and really, webmail is not the ideal place to transfer emails to. You'll need to use a program such as Outlook Express or Entourage to keep track of all of your emails in an organized and professional manner.
We personally recommend that when you do use webmail, use Neomail. Neomail is a sleek program that is very simple to operate.
Horde is a bulky program that is buggy and does not get updated very often, and overall is an unstable program. However, some people seem to master it and be able to handle it, so we still have it there in CPanel in case you are one of those people.
Normally we do not move accounts into your reseller account, that task is up to you. You are in charge of creating your client's accounts and helping them move their files properly.
If you ever need a database that was previously on another server, installed for one of your clients or anything that you aren't sure you will be able to do correctly, do not hesitate to contact our support address.