Does NEM supports atomic swap with BTC?

Not in its present form and probably not in the future either.

There will be atomic swaps between the NEM public and private chains and its child chains like Proximax when the Catapult upgrade is complete but I've seen little mention of crossovers with totally separate chains.

https://webcache.googleusercontent.com/search?q=cache:cQABcpXz0lEJ:https://nemtech.github.io/guides/transaction/using-secret-lock-transaction-for-atomic-cross-chain-swap.html%3Fhighlight%3Dnem+&cd=23&hl=en&ct=clnk&gl=uk&client=opera

https://nem.io/catapult/

Stuff like LTC has the luxury of being based off BTC and being fully interoperable. XMR is planning on adding the features needed for atomic swaps with other chains. NEM is 100% alien to them.

I think Catapult will allow decentralised exchanges but it's not quite the same thing.