BGLAnalyze and NewBGLAnalyze can be used but won't always work, especially with FSX. After editing, use SCASM to recompile the BGL. That doesn't always work as well.
Do you want terrain or objects from FS2004/FSX? The terrain files won't display properly.
Some add on objects from FS2004 will work in CFS2. They won't be destructible. Airfield backgrounds usually don't work as they are not bitmaps in the civil sims.
Objects from the civil sims can be converted into destructible GSL objects for CFS2.
There are various sets of ground textures which come from FS2002/FS2004/FSX available for CFS2 use.