It is possible for a workflow rule to process multiple values, but not directly - create a function module type rule and in it run over the lines of the table and call the responsibility rule for every line (use function module rh_get_actors).
This will result in many approvers for the workitem, but remember that this will be only one workitem, when one of the approvers completes the workitem, all of the other users will no longer see it and the workitem will be complete.
If you need more then one approver use a 'parforeach' type block and in it use the regular rule for every line - this will create multiple workitem.