You cannot transfer characters to congested servers. That is much of the point of making a server congested.

Servers can be (and often are) temporarily locked for character creation. Prior to the introduction of the "Congested" status, Balmung was locked thusly nearly all the time but one could still transfer characters to Balmung - which people did, making it grow even more.

Balmung won't be forever congested. It has been unlocked and had the congested status removed a couple of times. Each time it was open for some weeks before it was declared congested again.