Is it possible to update probation end dates for single assignments when a probation is extended?

Summary:

As part of the performance management process, there is a possibility that a new start may have their probation extended. Is it possible to update an employees record within impacting others in the same position?

Content (required):

We need a means of updating the probation on an employees assignment to accomodate any extensions which result from a probation review. Currently probation end dates are auto calculated based on the probationary period specificed on the postion and the hire date. In instances an employee probation end date needs to be amended, how is this possible without editing the position itself (which may have multiple incumbents)?