In order to create your own startpos files, you need the assembly kit for Rome 2. Please note these files are just to create the startpos not to use to play the game.
1) Download the following files:
Kit Files - https://drive.google.com/file/d/15Oa...ew?usp=sharing
Build Pack - https://drive.google.com/file/d/1hpOJuI4jCZAcTEv0tHC0znRpGBszk0Li/view?usp=sharing
2) Unpack the kit files into the assembly kit directory under raw data. This is usually something like Total War Rome II/assembly_kit/raw_data. You will be prompted to overwrite, say yes. You are overwriting both the /db/ and /EmpireDesignData/ folders. If you have other work you are doing, you should backup these folders first.
3) Place the build pack in your Rome 2 data directory.
4) Make any changes you want to in the kit files. Make sure they are also reflected in the build pack if you add new features.
5) Export the startpos using BOB to compile it as you normally would.
6) In the startpos you have to manually change this value to True to get buildings to show up in the tech tree: