I don't think you can use FN_FREE slots. Without VI expansion you can't do anything much.
You can however make some units and buildings available for a faction. The two files you need to look at are crusade_unit_prod11.txt and crusade_build_prod13.txt in the root folder.
The problem is, again, you can't make a new faction. What you can do is replace the existing factions. I usually replace the Burgundians.
I'm not familiar with modding the unit graphic so I can't help you with making a new unit.
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