Using NX-5...An ellipse is created using curves with parameters. The ellipse might be faithfully added into a sketch using the add curves function and the centrepoint positioned with dimensions, while the semimajor and minor can also be dimensioned.
Assuming that you're not using anything older than NX 3, the attached example should work for you as you have complete control of the size, angle and origin of the sketched Ellipse.
John R. Baker, P.E.
Product 'Evangelist'
NX Design
Siemens PLM Software Inc.
Cypress, CA
A small tip ... when you ask a question be sure you mention what version of NX you are using.
On here the versions typically range from NX3 to NX6, as you can imagine there are differnces between them.
Good luck !