You can’t add them as additions for this year if you have already been using them to earn rental income in previous years. You also don’t need to go back and REFILE your old returns. But you would have to calculate what the undepreciated capital cost (UCC) is to claim CCA (capital cost allowance) this year.
This Canada Revenue Agency (CRA) page has an example of the calculations for a rental property: Example of capital cost allowance (CCA) calculation
You would do the calculations for each year until you have the ending UCC balance for last year, which would be your starting balance for this year.