Tabs in a separate file from a book

You can add them to the Indesign file itself if you want - InDesign can have multiple page sizes.

If you wanted them in a Book file then each tab would need to be a separate document. 

Personally - do you need them in it for any reason? For print I'd supply the 2 files separately and let the printer know where to insert them - they can sort out the rest.

When I did prepress and printing myselft this is how I handled and preferred files. 

But even when you make the 2 pdfs -  you can use Acrobat to insert pages from another document.

Recap of options

1. Combine them in the original InDesign file - https://community.adobe.com/t5/indesign-discussions/how-to-move-pages-from-one-indesign-file-to-another/td-p/2317236
2. Make each tab document a separate file and compile a book and insert the Tabbed pages between your sections. 
3. Combine them in the PDF after the fact.
4. Leave it as it is and request the printer sort it out by clear instructions.