What objects are you refering to?
turrets? ship parts?
In case of ship parts, union (turn off all the options in the union window - triangulate first and the other two options)
This is VERY important, for you will most likely need to add a few vertex manually (it's very easy actually, but the second option in the window I metioned will delete all hand-made vertex when you union/substract - and that's something you want to avoid!)
If it's turrets, lods, debirs an such, then do the folowing (you can convert a ts .scn file to pof) - using a small plugin (darn..I can't recall the name.....used to view hierarchy..) you just drag and drop first all the turrets and then all derbris onto LOD0...
If you have some turrets for LOD1 drop them onto LOD1...Don't touch the other lods...
Bfore you save and convert make sure all LOD's have normalized and centerd axis( do move a axis a bit if the lod's don't fall in line), and that they are all at (0,0,0)...