So I'm trying to make the Civilian Transport fire a projectile weapon. Every time, it fires from the origin, not the weapon attachment. How can I change the location the unit fires from?

Not all units have all attachment points, if you look at it in the model previewer you can find a list of all attachment points the model has.

Out of curiosity: how can we manually add an attachment point to a unit's model?

Edit the model in an external program

I already looked in the previewer. That's the first thing I did. But how do I change WHERE the projectiles fire from? It should fire from the end of the units turret at 'weapon', but it's firing from 'origin'.

In the Action actor for the attack under Container Attachment Query you should have the weapon filter there. If you do not have an action actor for your attack you should, there are plenty of examples there (you could duplicate a similar one if you wanted to).