The non-degree program is open to academically qualified persons who meet the same admission requirements necessary for the MS in Data Science degree program. See here: Admissions Requirements
Non-degree students are permitted to apply to the degree program (please note that non-degree students may apply for and, if accepted, begin the MS in Data Science program in fall semesters only). A new application may be required at the discretion of the admissions committee, and there is no guarantee that they will approve the transfer.
Please note that international applicants applying to the non-degree program, must have a visa. Please see the GSAS site for more policy information: NYU GSAS Visa Policies