There's no notion of asset expiration inside MultiChain itself. But can you explain what you would want asset expiration to mean? If you mean it cannot be transferred any more in MultiChain transactions, then you would need to look for your solution elsewhere. But if you just mean it no longer has value and cannot be redeemed, then this can easily be handled as the application level. You can use an 'open' asset to allow the original issuer to create further issuances that update its metadata, setting the new expiration data, but add no new units.