So if I’m understanding correctly, you want to save all components (of a similar type) into one Skp file. So all bathroom sinks in a file, toilets in another, doors, etc. That can work, I’ve also tried that before, until I had too many (due to too broad of a category). I suppose if you plan it out your categories in advance, you can above having too many components in one Skp file.
You have each component as a separate Skp file, but categories them into different folders, a folder for just bathroom components. The component browser can have work in multiple folders I believe.
If you’re trying to get the best of both worlds, all components in one file and viewable separately in the component browser, I don’t think that’s possible.
As for your original question on saving out a component to an existing file, I’ve never seen that before, don’t think it’s possible in a single action. Perhaps with the ruby script, you can script it so that an existing file will load all components from a folder where you’ve saved out all your components separately. Anyways, good luck!