You can unlock it for staking when its open but once you have an amount reserved for staking it will go towards staking on the network anyway.Having an encrypted wallet wont stop you earning POS rewards.
You could always import the private key into another wallet.dat.

I would just slide out the encrypted wallet.dat from executables and let it run and generate another wallet.dat and then import the private key from the encrypted one if I wanted an unencrypted one because you cant remove the encryption from one already encrypted.