Hello,
I have a client who is interested in hosting with me, however, they currently have a account with One World Hosting, a Alabanza reseller AKAIK. Is there any way for me to transfer the email accounts from them to my Cpanel server? I noticed there is a transfer feature but it needs a root password. Is there a way to do it without a root password? Re-creating 100 email accounts would be a very painful process.

Just have him change the Pri DNS i.e. ns1... to your server. Make sure you have all of his files on your end, there should be very little to no downtime...

Opps, I re-read :) ... <<--- Windows Host , I have no clue hehe

Ok, but is there a way for me to transfer his email addresses without the root password to his server?

all of the email account names are stored in /home/$user/.mm/$domain.mailaddr.

all of the email account passwords are stored in /home/$user/.mm/$domain.passwd.

I'm not sure of how email accounts are defined with CPanel but I'm sure that once you know that and you know the info above, it can be easily scripted.