cropop is right that it is university specific. Sometimes it's at the department's discretion, sometimes it has to go through the university for approval.
When I got my acceptances I wanted to defer for a Fulbright ETA, and some departments told me yes but the fellowship couldn't be deferred, another was willing to be very flexible. Having a student who gets grants from an organization like Fulbright looks good for a department, so that also might have helped. The best you can do is just ask the department. Every field might be different, but all of my schools were extremely helpful and looked into the options for me.