The most efficient way to add edited volumes and chapters to Zotero is to import them using the Zotero browser connector from the publisher webpage. You should very rarely need to enter any item details like this manually.

Thanks for your answers.

@bwiernik:
Unfortunately in my topic, I often have to enter them manually and cannot use the browser-plugin.

@adamsmith:
In the technical handling, edited volumes may be not very different from books, but bibliographical they are very different. And even if bibliographical standards are ignored, it is not a good usability to change the authors to editors manually for every edited volume. And it is also not good usability, if you have to search the relevant entries for the articles among all the entries that are only relevant for the corresponding edited volume. E.g. I search for the pages-field every time.

We have to admit, that this is done much more comfortable in Citavi, where edited volumes and the articles in them are handled differently than monographs.

Again, I agree with the second point -- having chapters better associated with edited volumes would be great (though note that Zotero, when using "Create Book Section" does "relate" the chapter and the book, so they're not entirely disconnected and you don't have to search through the whole library). I'm just saying it's unlikely that'll happen any time soon.
I am puzzled, though, what sorts of books you are entering that don't exist in any library catalog? Do you have an example?

Where I disagree is that edited books are "bibliographical very different":

In all citation styles I'm aware of, both German and English, the only difference is the addition of ed./Hrsg. in some form. Otherwise they're cited the exact same way (which is why, as bwiernik points out, metadata standards treat them the same; libraries, too, btw -- it's very rare that you can distinguish in a search/filter between monographs and edited books)