There are opportunities for post graduate studies in the CF. The numbers that might get to do so will vary between classification, and can vary from year to year depending on available funding, strength of the classification against establishment positions and the needs of the service. It will never be a high percentage of the available numbers, and may not be offered every year in a particular trade.
Most that I have seen selected for post graduate training have been experienced Captains, Majors or more senior officers. Alternatively, if you are identified early as being particularly suited and the trade has a need for officers with the program you are pursuing, if could happen earlier.
Selection of the program of study can be up to the individual, but you could expect the technical classifications to be more supportive of people wanting to further their education in that field, otherwise the cost of letting you do the studies doesn't exactly benefit the trade.