AC 61-135 describes the procedures and processes involved in converting the FAA pilot certificate to Canada TCCA pilot license and vice versa. Conversion does not mean that one has to surrender the current FAA or TCCA pilot certificate. It is not authorized for a pilot to obtain and maintain both FAA and TCCA pilot certificate or license. The FAA Advisory Circular describes the procedures clearly.
In a nut shell, a pilot medical examination and a written test (multiple choice) or 2 are required for the conversion or addition. Download the AC 61-135 and review thoroughly. If you have any questions about this, feel free to post them in the comments section below.