Can I add devices offline?

I just setup my first home assistant server using Raspberry Pi 4. I am away from home for a few months and wanted to get a head start on configuration.

Can I add devices such as Phillips Hue and SimploSafe without having them on my network? Sort of like a “ghost” device?

I assume you meant SimpliSafe?

In short, the answer is no.

Configuration of both these integrations rely on discovering devices on your network.