as the other topic says: the real solution is in scribus 1.5.
there you can put a master page layer on top of a page layer... you cannot do that in the current stable scribus 1.4.
as a workaround, you can
- create a frame with the page number sign in it
- correctly place it in the current page
- send it to the scrapbook
- open the scrapbook and keep it open
- click on each page to select and double click on the scrapbook item to place it at the "original" position
of course, you can create two scrapbook items, one for the left side and one for the right side...
a bit tedious most of all if you change your mind afterwards, but you should manage to add all the page numbers in about a minute...
ciao
a.l.e