It is impossible for resellers to use the transfer functionality. Account transfers is simply not available within a reseller's WHM.
So, you have to do it with root. (If you have it!)
However, root doesn't allow you to import accounts directly to a reseller account thusly it won't honor the dedicated IP nor nameservers associated with that reseller account. That means that:
1.) You need to manually modify nameservers in the DNS zones 2.) Need to manually point source -> new server as for DNS zones 3.) Need to manually re-assign IP's.
Needless to say that takes tons of time...