HaloMods "Classic" Read-only Archive

I am having dificulties trying to import objects into other maps. I can not find any tutorials on doing this. Specifically, I would like to put a flat block in boarding action so I can make catwalks. More generally, I would like to be able to import all sorts of objects that I create into any of the maps. Is this possible? Does anyone have any PPFs for boarding action (or other maps) that include building blocks?

Thank you.
Ollie