Can unlock work with existing NFTs?

Say for example, I am a owner of a OpenSea collection like https://opensea.io/collection/cryptofoxes-origin

Can I use those existing cryptofox NFTs as the unlocker, instead of generating new NFT tokens?

No , It is not possible to use the existing NFTs as unlocker
One work around can be to generate new NFT tokens using unlock protocol and airdrop to the address who have cryptofox NFTs in their wallet