There are several third-party 'raster-to-vector' conversion utilities out there and that would be the best route to go.
As for inside NX itself, the only practical approach would be to import the .png/.tiff image and then 'trace' over it using 'sketch' or freeform curves.
I have very little direct knowledge of the capabilities of the new 'Convergent' technology tools included with NX 11.0 but from what I do know, I think that it's limited to 3D models, particularly working with lightweight/faceted models imported into NX and then easily incorporated into traditional solid/surface modeling workflows. I'm not aware of anything specific for the conversion of purely 2D graphical data such as found in an image file.
I would suggest looking at Inkscape to convert a raster image into vector.
I made a video a few years back which explains how to do what you're asking: