I've finally been able to get this working and wanted to post it here for reference. Thanks to Perry for providing a lot of the advice and explanation which made figuring all this out possible.
Anim8or is a free 3D modeling (and animation) tool. I was interested in creating 3D objects to place in my FMS landscape using this tool and found the below to work well.
- create the object in the object mode of Anim8or and use 4-sided quad-based geometry (ok to have other face types, but they won't show up in FMS)
- export the object as a Wavefront (.obj) file
- use the wf2scn converter tool I wrote to convert the .obj file to an .scn file
- paste the contents of the generated .scn file into FMSXYZ, save, reload, then drag to position your object and save again
You can download the wf2scn converter tool executable and source code here.