This is not supported by the paging toolbar. You'd have to modify it in order to attain this behavior.
The first step would be understanding the source code of the paging toolbar
as of Ext JS 3.
to what you'd want.
Unfortunately I couldn't find any specific documentation on doing this for Ext JS 3, but I seem to have found someone with the same questions back in 2007, that got an answer; this hopefully would help you attain your objective: enhance PagingToolbar.updateInfo to add selected rows
I hope this helps!