We are creating spare part manuals in InDesign where some of the pages are for only a few different parts where we want to change their page numbers as for example from 7 to 6.1 (because page 7 is originally a slightly changed version of page 6) and then continue to count the main pages from where we left. This is not applied to all the pages but just to a subset of them. Is this possible and if yes how can we achieve it?
You would do this with section starts and prefixes.
Open the Pages palette and double-click on page 6 to make it active. Open up the flyout menu of the palette, and select Numbering and Section Options.
Leave the Start Section square checked. Click on the radio button which says Start Page Numbering at and put in 1. Yes, one.
Under Page Numbering, there is a Section Prefix line. Put in 6- or 6. or whatever you want it to be.
This makes all the subsequent pages 6-1, 6-2, etc. or 6.1, 6.2, etc. as you prefer.
When you come to the end of the 6- pages, you click on the next page, make that a section start, and under Start Page Numbering at, put in 7.
(As a separate tip, if you find difficulty in printing or exporting a PDF because of the section starts, count your physical pages and use a plus sign before the number to indicate an Absolute page number. In this example, +7 would give you the page named 6-2, not the page named 7, because that could be 10 pages farther along.)