I bought the game from Focus, not Steam. Can I buy the addons from Steam, tho? I wanted to buy the game on Steam, but there was a small problem with my account at the time and I had no way of knowing how long it would be to get fixed.
You can't install them on a PC that does not have an internet connection.
You can't download the installer by itself.
If for some reason Gamersgate goes out of business, and I have to re-install, I will be screwed. I realize the same could hold true for Steam, but I think it is less likely for Steam to go under than Gamersgate.
Can I download the game on my office PC and install it on my home PC which does not have an internet connection?
No, you cannot. Internet connection is required to install games. If the target computer has slow download bandwidth, then the game can be downloaded on another computer and then transferred in any way to your original computer. There you can use them to install, but connection is needed to sign in and start the installation process.