Questions from a newbie

I'm new to Composr, having just started looking at CMSs.  I'm in need of replacing several different story archives with a single CMS solution.  So far from what I can tell, Composr meets about 90% of what I'm looking for, but I need information on that last 10%.  Hoping I can ask that here.

Multi-chapter like capabilities:  Is there any way to tie two or more entries together?  I'm coming at this from a fiction archive, and lots of fiction has more than one chapter.  Ideally, I'd like it if someone could pull up their story and somehow "Add chapter" to add in another piece.  Is this even possible, perhaps with an addon/plugin?

The drop-down additional fields that I've built do far will help with most of what I need.  But there's the possibility of entries in a field not being availble, e.g. a person creating an entry may pick "A" and "B" from the dropdown list, but needs "Y" to be an option.  Is there a way to have a person pick A and B, and then free-text Y?

Hi,

Well, the chapters sound like entries in catalogue categories. So you'd have a catalogue called books, and then each category in there would be a book, and each entry in the category would be a chapter.
It'd be tricky to limit individual users to their own categories though.

Regarding catalogue list fields, there's a 'custom_values' field option you can set to 'on', which lets people type a custom value instead of selecting a list entry.