You can add forts at the bottom of the descr_strat.txt file, just scroll down to the region section...

The first part looks like

region Britannia_Inferior
road_level 0
farming_level 0
famine_threat 0
watchtower 53 152
replace watchtower with fort thus:

region Britannia_Inferior
road_level 0
farming_level 0
famine_threat 0
fort 53 152
I think the ownership goes to the one who owns the region, but I have not investigated this. Perhaps you could get back to us and fill us in? I hope this in part answers your question. Yes, they can be built before the game starts ;)