not on the base layer and only via rounding on lightning (you don't want to have to round with smaller and smaller denominated units with final settlement) https://lightningwiki.net/index.php/Denominations
"The amount is rounded down to the nearest satoshi when the channel is closed and broadcast to the blockchain to adhere to bitcoins specification."
It'd require a hard fork to get millisats on chain, but I'd support it if it were needed.
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