How to add a building?

[Agent Smith]

Unpack the strings.bin file using alpaca's converter and add the line in. Then let the game self convert it back to a string bin and it should work.

So it will look at the text file and then overwrite the old strings.bin file? If that's it, then maybe I'll be good to go!

[FactionHeir]

Yes, pretty much. You need to do it in a mod folder though, not with a file_first only.
Check alpaca's basic mod folder topic I think it was for details.

[alpaca]

You can use the mod folder installer (if you have 1.2) and uncheck the launcher and music settings, that'll give you most of the text files already converted and ready to be edited.
You can also use that mod folder to generate your strings.bin files if you want to use io.file_first