Yes, it is possible to only include certain documents into an index. Page ranges, no.
- Create a second book file with the documents you want to index. Exclude the ones you don't. Be sure not to repaginate/synchronize the second book or apply any page numbering options. You want the index to recognize the pages as they would be in the full book file.
- Run your index, and get the text file you want for your separate index document.
- Put it in a unique InDesign document and then make index document the last document in your full book file.
This should get you what you need. If not, come back here and sound the alarm. there are lots of sharp folks around here who may be able to help.
Good luck,
Randy