In looking at the NTG and Iron Rattler conversions, not much was done to the pre-existing structure that did not require modification for new track (such as changing large banked turns to overbanks, etc.). As far as I can tell, steel cross members were built to connect track to the supports. For new segments and elements that were built from the ground up, RMC still stuck with a wooden support/steel cross member design, such as Iron Rattler's top-cliff s-curve section.
[youtube]http://www.youtube.com/watch?v=X0ecL2ecVpo[/youtube]
[youtube]http://www.youtube.com/watch?v=arqSnM9T_Bk[/youtube]
With the Twisted Colossus conversion, while the original structure is wooden supports, the rolling corkscrew/stall section did feature new steel structure, which was built from the ground up for the new segment:
Storm Chaser was a unique conversion as it was already a steel support coaster - RMC built new support segments for the corkscrew and some turns, but all of this blended together with previous structure from Twisted Twins (of which, Storm Chaser uses supports from both sides).
[youtube]http://www.youtube.com/watch?v=_eKn6r1WBzA[/youtube]
All of this is to say, thus far, RMC has used whatever support structure had been used prior with I-Box conversions, even when building entirely new track segments.