Using simulation proofs
Anything is permitted as long as it works. The question you have to ask is what role a symmetric encryption plays in the protocol. If it is decrypted at some stage in the protocol potentially to a corrupted party, then you need to be able to simulate that and have it decrypt to the correct value. If it is never decrypted, then why is it ever used in the protocol?

Sorry for the self-citation, but you can read more about this at: https://eprint.iacr.org/2016/046.pdf.