John, Like Bradford said ....Only use a one piece length of tubing in the original size... 3x3 or 3x4 or whatever the original tube size was. Yes, it's possible to lengthen it by welding, but you'll destroy the galvanized coating at the weld point and invite rusting at what is arguably the most critical point on your trailer. (Don't forget it will rust from the inside out, so you won't see the rust until the tongue buckles and snaps off).
And, Like Phatdaddy said, you might be able to just slide the winchpost forward a bit. Just be aware that you'll change the loading on your tongue as you move the boat closer to the front of the trailer. You may have to adjust the wheel carriage to compensate. Remember (as a rule of tunmb) you want 10% of your boats weight on the trailer tongue for stable towing.