You mean an external inline reactor?
On the outlet side of canister filters, the flowing water is under pressure. That has two consequences when connecting CO2 to it: 1) The connections need to be very very airtight, because the CO2 needs to push against the pressure of the filter output and 2) If any connections fail/break, the content of the tank is emptied into your beautiful house.
On the inlet side, things are different. There is negative pressure, which can cause soda bottles to collapse, and empty the contents into the tank.
For these reasons, using a different method to dissolve CO2, without any pressure involved, would be preferrable.