Is it okay to defer accepting a PhD scholarship to see if I get accepted into foreign Universities? If I apply for a PhD scholarship at my current university, where I'm completing my honours, they'll notify me within 6 weeks of the deadline November 1st.

However, other universities I'd like to apply to overseas won't tell me until as late as March next year.

Is it acceptable to defer acceptance until I get a rejection from my preferred university? I really don't want to come across as ungrateful for the opportunity.
There are some variations between countries, but generally when you are awarded a scholarship tied to a PhD program the scholarship is automatically given to you as soon as you enroll. So you cannot defer accepting a scholarship. So you should figure out how the scholarship is paired with the admission process, if you have to enroll within a certain time of having received the scholarship that that's your deadline. You could also enroll and then drop out but in some cases you might have to repay part of the scholarship back, although unusual this could happen. Keep in mind that there's also the ethical side of things, if you don't accept the scholarship can go to someone else, and by dropping out after the program has begun you take an opportunity off someone else.