There is a solution:

The Collada export from Creator is available since version 4.0(?):
It converts all XRefs and transforms the geometry from world (Z-up) to screen coordinates (Y-up).

Since Unity does not support the SGI texture file format (.rgb, .rgba, .int, .inta) you have to convert your textures before the eport to a supported format (e.g. TIFF) and replace them in the texture palette.

Material generation does not work for the Collada files exported by Creator because of an old file format (COLLADA Schema 11/2005).

Workflow details:

Presagis Creator:

• combine geometry with the same texture into one object (or more depending on size, rendering issues ...)
• rename these objects
• convert all textures to a supported format
• use the same texture palette in all XRefs
• reread all XRefs
• export as Collada Document

Unity:

• Import Settings: Materials Generation - OFF
• Create new materials by hand for each texture
• apply materials by hand or use Mass Set Materials

The Z-up to Y-up conversation results in a rotation of 270 around the x-Axis.

The OpenFlight format specs are publicly available, so you could write a parser in Unity that converts OpenFlight files to Unity models and terrains. Doing so will be highly non-trivial, however.

The Collada export from Creator is probably the best option if you have a copy of Creator. If not you could always try Okino's PolyTrans with the DCC pack. It will allow you to import from most 3D file formats and export to just about any other. It will also convert animations and most 2D texture formats as well. It is a windows only product but for most things it will work with parallels or fusion windows emulators if you are on a mac.

unity supports many formats and you should export your model to one of those formats. FBX, OBJ, collada .dae and ... see the import section of manual