It maybe difficult - I don't recall seeing many cases of ventilation systems within the station proper - whatever vents there are seems to be part of the tunnel itself. Some of the stations also have little plenum space.
AoD
Most of the older stations have a vent shaft in the tunnel, immediately adjacent to the station, they rely on this, and the tunnels, and in some cases additional airflow (ie. Queen Street uses the former exit on the North-East corner as a vent).
There is little built-in ventilation in most of these stations.
That said, on Line 2, and the northern portions of Line 1, there is ample room w/station boxes, in most cases, to accommodate such a system, and vents mounted on the top of a PED system could be used to distribute flow at the platform level.
The original line one stations would be more challenging to fit out due to a much smaller box and lower ceiling heights as well in many cases.
If adjacent vent shafts have sufficient capacity, including meeting their existing obligations, it might possible to fit a/c units inside these, and then pump in, along the top of PEDs again, but I'm not clear that that is feasible w/o expensive expansion/alteration.
I do, however, think it really is an important investment over time. Overcrowding can occur any time, even w/an RL due to large events or delays, with overcrowding and overheating you run the risk of medical emergencies, taking both the toll on the people and the system.
Climate change models indicate we will see more extreme heat in the future and moderately warmer days throughout the typical summer.
We should be proactive, rather than reactive.
That's not to say there aren't higher near-term priorities, but the problem in saying as much is that that will likely always seem true, until something goes terribly wrong.