Hi,
This topic recently got indexed by Google (not sure why just now), so I thought I'd add some relevant information:
A much simpler approach to getting a RoC-only editor is temporarily renaming
C:\Program Files\Warcraft III\War3x.mpq
to something different. (eg.: War3x.mpq.bak or _War3x.mpq)
Then only the RoC editor will load, with all the RoC data.
Export the catapult, close the editor, undo the rename operation, open the now TFT editor, and import your catapult data.
(Or just take a screenshot of the RoC editor and copy the Catapult values manually. Sounds much simpler.)
--3ICE
p.s.: I discovered this trick while making 3ICE's Maze Designer RoC-compatible.