Here's what the maker said: "The welding is steel>steel. I like to do TIG because I usually do it in situ on the bike and TIG does't spatter and looks neat. But other forms of welding can be applied also of course."
Thanks MM, I had missed that. If you are going to weld it I think I would still recommend having a pro do it. Improper welding can significantly weaken surrounding metal and change its characteristics substantially. Especially frame welding. The heat cycles can change the malleability of the steel and create brittle spots that become prone to failure but aren't visible (from experience with roll cages). Also, although both are steel they are very likely to be very different kinds of steel with unique characteristics that should be taken into account with the depth and temperature of the weld. Just some thoughts.
Also, if your an iPad user I highly recommend this magazine (available on the newsstand):
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Recent articles include mods such as this.