The only control you have over the selection in the datagrid on page A is if the first row is selected. So from page B you could determine what object you would like to select on page A. Change a sorting attribute on the object for page A, so that this is the top record in the datagrid and make sure the datagrid is set to select first = true.
It not a pretty solution but might work, you do however need to take into account that multiple users might be doing the same action, so the sorting needs to be linked to the user performing the action. Also think about a reset when pressing the button from page B.