Hi,
The system works on the basis of "importing" so that the program knows where to find the stuff you want to use - you can't duplicate or add new files made in other BC or other programs straight off.
However the import buttons on most of the BC menus don't work the way one would expect. I've forgotten how I did it with Army Templates, but with terrain, symbols etc you use the Edit menu, then components, then new, look for "Fill Type", and adjust to "image". This gives you a route menu just up from the bottom where you can find the image that you wanted to import.
That makes your terrain or symbol imported. So that the file can be selected in future when creating a scenario. I try to use maps drawn in Hexographer and imported into Battle Chronicler. Despite setting the terrain for x feet by y feet in BC, the resultant file in BC isn't stretched to fill that area. I've not got that far.
Importing army templates. I can't remember the details, but seem to recall it being related to me creating a scenario, then finding the army file i wanted to import, linking it to the scenario. Hey presto that army template appears option across the rest of the software. However, It's been a while and I've forgotten the procedure.
Hope this helps.