Is there a way to create ossia staffs that only show at some measures?
You cannot make staves appear and disappear.
For other discussion on this, use the search facility to search on ossia.
This has been discussed in previous threads (not always using the term "ossia" for search).
The bottom line is: Although that's not a built-in capability of NWC 1.75, you can achieve the effect by exporting placeable WMFs from file > print preview > copy, then place them into a word processing document, then crop them. It's not hard.