Its easy enough, worked fine for me;-
- Open .bse file in WorldBuilder - when you click the 'Bases' tab, you will only see .bse files that have been placed in the Bases folder relative to the BFME directory, otherwise you will have to change the file extension filter to 'All files';
- Make any edits or changes;
- Save the .bse file to the Bases folder.
Each side has a different .bse file depending on its eventual orientation in game (which in turn depends on whereabouts on the map it gets unpacked).
I did it to give Mordor and Isengard fences around their 'castles';-
