Basically, the reason you can't is because the right half of the pager doesn't contain the table used to order the buttons, so it has no suitable place to anchor the buttons. It's a bad work around, but adding the line

$("#jqgrid_toppager_right").html('

');
will let you add buttons to theright half of the top pager. This creates the basic table inside of the right half, which will allow you to add buttons to it.