you can do it, but it is very expensive to do it right. not only do you have to worry about the top, there is also a lot of stiffness that needs to be added to the car. to do this, you would have to take the whole car apart (among a whole bunch of other work)
you are definitely better off spending the extra money toward a newer, more improved nsx.
from previous discussions, the general consensus is to buy an already-targa nsx.