
Hei Pei.

I can not help you directly width you problem, but there may be
intermediate roads to take to get to the IFF. I am using a converter
that can take IGES, IIF, DXF -> IGES, MILESPEC I IGES, MILESPEC II IGES, 
IIF, MILESPEC I IIF, MILESPEC II IIF  and DXF.

IIF is IBM IGES FORMAT. There may be converters out there that can handle
IGES to IFF. Hope this was to any help. By the way the converter is part
of the IGES Processor/6000 package from IBM and it runs on RS/6000 AIX.