Are they blocked by the Paid Character Transfer system?
There isn’t some separate function called an Account Transfer. It’s just the ability to select another active account before picking the realm to move to. NOTE: Active is important. If you don’t have game time on the destination account, you can’t even make the transfer in Retail.
EDIT: Looks like this was asked and answered in Customer Support as well. It is a specific limitation to WOW Classic for now.