A lot would depend on which sim the scenery is intended for & which software you're using to create it. It's possible to convert the less complex FS98 aircraft models into scenery objects, either direct to BGL format or as API macros which can be imported into a scenery design program like Airport for Windows or FSSC. However, things have moved on & I'm not sure these programs are used very much these days. If you wish to try it the original utility is M98toBGL posted here. --> http://www.simviation.com/fsdesign_scenery5.htm
The updated MDLtoBGL is still available here. --> http://www.avsim.com/hangar/utils/mdltobgl/id2.htm
Instructions are included. Both work best with very basic aircraft models. There are plenty to choose from in the FS98 sections.

Since AI traffic was introduced with FS2002 I prefer using that method rather than static scenery objects.