In the correct environment, I believe they originated in such places as the Australian Outback, they have their uses... but as said above in urban environments such as in this country then they should be banned...
However, enough of the lecturing, as we don't know why you want them, I don't know the answer to your question... I'd suggest that you try a Forum which deals with 4x4s rather than a general forum such as this, they'd probably know!
x trail - bull bars/nudge bars - Dwight Van Driver
Yes it is possible and fraught with danger. If other than recommended by Nissan ( and they don't come cheap) then possible offence of parts and accessories likely to cause danger = fine and 3 Points.
As a fitting then it is a modification and as such Insurance Company HAVE TO be informed. They do not like bull bars.
There was a move some time ago to prohibit them but things seem to be dormant.
For normal road use, why fit bull bars? They cost you money, add unnecessary weight to your vehicle, spoil its appearance and they can be dangerous to pedestrians.
If buy posh chromed bars to protect the bodywork you'll probably find yourself worrying about getting the bull bars scratched or bent instead!!