Not with the site navigation bar, at least not that I am aware of. But you could use a sidebar and manually make a navigation panel by embedding links to the pages you want. It would look similar to the default site navigation except it would give you the flexibility to have links to pages a,b,c from one page, and links to pages x, y, z from another page. When you use the default site nav it is the same for all pages on your site.
There is a video on my site that shows how to link to content. You'll be doing something slightly different but watch it and you'll get the idea:
https://sites.google.com/a/richland2.org/howtomakeagooglesite/home/linkingtocontentWatch from about 1:30 on. The difference -- when the link to dialog box pops open, select "Sites page" and do not check to have the page open in a new window. This will function exactly like a nav bar but you can make it page specific.