If I understand correctly: No need for scripts.

If you can't find the actual origin: SHIFT S -> Cursor to Selection, then look for the cursor. You can also press N and check the object's location.

If you want to set the origin to the center: Rightclick (or press W) and select: Set Origin -> Origin to Geometry

If you want to find the median of all objects' locations in the FBX: select them all and again SHIFT S -> Cursor to Selection.
You can press . (period) and also choose a different center, like bounding box.
After doing this, you can also set the pivot of all objects by selecting them and Rightclick (w) Set Origin -> Origin to 3D cursor.