In theory you could have a fraction system of satoshis allowing you to eventually materialize sub-satoshis and second could be cryptocurrency that is pegged to Bitcoin and has a smaller unit of account than a satoshi. This new cryptocurrency could be used for LN transactions, allowing for sub-satoshi transactions without requiring changes to the Bitcoin protocol.

This would mean a lot of changes to current infrastructure and it's theoretical "how-to". Not saying it's good or bad.