There are no UDA's in NWO365 apparently. I haven't tried myself to develop in O365 yet but that is what I read all the time. There is more to migrating from 2010/2013 as you might want to read in this excellent post. I am pretty optimistic UDA's might show up in a new version of NWO365 some time.

Thanks for the Reply.  I wonder why there are UDAs in Nintex Exchanged marked as for O365.  Disappointing but I found a method for what I was trying to achieve.   

Dear Nintex team,

We are in the planning phase to migrate SP2016 to SPO and we would like to know your recommendations. How can we migrate UDAs(site uda/ global UDA: central admin)?

Is this feature is available in SPO now?

Regards,

Deepak Singh