Yes and No. You can not include amounts for your Private Health Services Plan (PHSP) or Extended Medical/Dental that relate to prior years on your current year tax year, unless you choose to use a "one year block" that ends in the year you are filing.
For example: If you are filing the 2018 tax return, you could choose to use the period from July 1st, 2017 to June 30, 2018 as your one year block.
It would probably be easiest to make an "adjustment" to any prior years that the amounts relate to and base it on an annual basis.