If your customer has an environment that can't setup Dirsync (perhaps they are not on Windows Server 2003 DC and above), and you would want to create users manually, here is what you should do:

1. Create a CSV and use Administrative Portal | Users | Add Bulk Users to import the users.

2. Export the X500 addresses from Exchange 2003 server (You can refer to this link on how to do it).

3. Import the X500 addresses that you have exported from your local Exchange 2003 to Office365. You will have to do the following in Exchange Online Powershell:

$foo = get-mailbox jabez
$foo.emailaddresses += “X400:C=Malaysia;DC=domain;DC=com”
$foo | set-mailbox

If you are doing a temporary co-existence without using the recommended approach (Exchange 2010 Hybrid), you may also want to:

1. Create mail-enabled users with external email addresses to @tenant.onmicrosoft.com. If you have 100 Exchange on-premise users, then you will have to create 100 extra mail-enabled users that points to the @tenant.onmicrosoft.com email account.

2. Create a forwarding from the Exchange on-premise to Office365 http://www.victoryag.org