Extending Probationary Period

Hello, 
I am in Manitoba and am looking for some information on extensions of probationary periods. I read in one of the HRInsider articles that an employer cannot extend a probationary period without the ee’s agreement and fresh consideration. 
What if you write in to your employee handbook (which everyone signs upon being hired) that your company can extend the probation period as long as they give written notice to the employee prior to the period ending?