Howto:Convert objects with ModelConverterX
This tutorial explains how to use ModelConverterX to convert objects from a number of other flight simulators. This tool can do batch conversion of models and their textures, if instructed so, changing also their name to upper case for better Linux (and other case sensitive OSes) compatibility. Thus, this potentially represent an easier alternative to Howto:Convert objects from X-Plane.
Note: As of April 22nd, 2014 there's not yet full compatibility with AC3D, so it might be better to wait for it to be fixed, to avoid that the converted models might not work anymore with future versions of FG or AC3D. More on that below.
Please note: objects that are uploaded to the FlightGear Scenery Database (and thus official FG scenery) must be released under the GNU GPL license. Most add-on sceneries from other flight simulators (X-Plane, MSFS...) does not comply with this license. Therefore, it cannot be included with the official scenery, unless the author granted you permission to release his work under GNU GPL, and you assured yourself that the contents he/she used are in public domain or otherwise GPL-compatible, or under his/her copyright. Basically, it mostly means that it is for your own private use only.