This is probably one of the simpler modifications, since the Temple to Vulcan is already available to at least one Roman faction.
First step, go to the data folder and make a copy of the "export_buildings" file. Then, open it and scroll down to "temple_of_forge". It will look something like this:
building temple_of_forge
{
levels temple_of_forge_shrine temple_of_forge_temple temple_of_forge_large_temple temple_of_forge_awesome_temple temple_of_forge_pantheon
{
temple_of_forge_shrine requires factions { dacia, seleucid, romans_scipii, }
You want to edit the third line, so that it looks like this (added part bolded):
temple_of_forge_shrine requires factions { dacia, seleucid, romans_scipii, romans_brutii, }
Make sure you have the comma and space after it inside the brackets. Do this for all five levels of temple, adding "romans_brutii, " to each of them. And that's it. One note: make sure you're editing the temple_of_forge, not the temple_of_battleforge (that's a different one).
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