I believe gparted is intended to extend the encryption onto the expanded partition the same as it does the file system. Note that I have not tried it so this is untested in my case.
The file system/partition must be expanded as a unit so the only way to encrypt it before expanding would be to create a new partition and that is not what you indicate you wish to do.
Maybe someone who has actually expanded an encrypted partition could chime in here with first-hand information.