The metal trans lines do rust out. If you take a look at the trans cooler from below you'll notice the cooler itself has flexible rubber lines running from it to the metal lines. Hmmm... Trace back the metal lines until you get free of the rust and crud, then cut the line where it is straight at for a couple inches. Replace the line starting directly at the trans cooler with a new longer flexible line. Slide a couple inches on the metal line where you cut it and double clamp. If you have access a flare tool could give you a little more security, but frankly the trans line it not extremely high pressure or tempature. The flexible rubber hose is sold by the foot at most autoparts shops. Just take the old line in an have them match up the inner diameter. Dont try to reuse the old clamps.
** Edit **
Think I misread your question. The metal line coming out of the radiator itself is leaking at the fitting? Well yes you can replace the steel line and fitting. It a double flare similar to a brake line, only larger diameter. Chances are pretty good that the fitting and line will twist to junk when you try to remove it from the radiator. If it unscrews cleanly without damaging the radiator, you will need to install a new fitting and at least a few inches of new metal line. Yes you can splice the line afterwards if replacing the entire metal one is not in the cards. It will work, but not the optimal solution.
