They haven't made an official statement as to whether they will credit SegWit2X or not to their users as far as I know.
But from what we have seen in the past, they will most likely do.
That was the case with Bitcoin Cash. You can also read their recent statement on Bitcoin Gold:
https://support.bittrex.com/hc/en-us/articles/115002320451-Statement-on-Bitcoin-Gold-BTG-Bittrex will make all reasonable attempts to credit our users BTG but until we have a fully supported wallet, we can neither guarantee tokens or consider creating a market. We will continue to monitor the state of the codebase and network. We'd also like to encourage the BTG development team to reach out to us.