Chris,
Check your yellow pages under welding or machine shops. These places often get steel deliveries on a weekly basis, so you can avoid special shipping charges and mark-up. they might even have what your looking for in stock. If you still want to use your trailer to haul other stuff you can strap a piece of tubing under the existing tongue using square U-bolts, then remove when you dont need it. The long tube will have to be taller or have a heavier wall than the existing tube to get the same strength. Check the tube size on the coupler you want before you buy the tube, most are 2.5 or 3" wide. You will also need a longer wire set to match or make a plug-in extension cord.
If you want to build a dedicated yak trailer I would start with a lighter platform (I used the 3-1/3' x 4' mini from H.F.) Take a look at AAA trailers web site for ideas on size and shape.
If you have to pay somebody to modify the kit and retail prices for steel, you might sink in more than store bought and end up with less. Worst case it fails at highway speed.
Good luck