Would it be possible to add these to an existing savegame? I started an angelbob game with whistle stop factories and am already quite a few hours into it. It seems like I would have to find new chunks to get these to spawn, but it would be nice to have some closer options.

If you want to add this mod to an existing save, I recommend turning on the crafting recipes in the settings, so you can make a few big factories yourself. The map has been generated a lot further than has been revealead, so it would take a while before you'd run into new big factories.

Note that there is no compatibility with WhistleStop, so if you uninstall that and install this, all your existing big buildings will be gone.

I was hoping there'd be a command line remote call to create factorioes into already existing chunks. For example RSO has an option to add ores to already existing chunks if you added a mod with new ores later on. Something like that for the factories is what I'd prefer, as I don't want to make them craftable. I play a islands map and building cargo ship lines to various big factories is kind of part of the point of the playthrough.

I'll consider adding a feature like that at some point, but I won't have much time soon. So that's a definite maybe.