Adding the needed high-voltage, high-current wiring might be the issue.
Forgetting to "pull the plug" on the battery and working on live circuits
would be ... very dangerous.
Yes, the QC would be "easy" to add if the accessory was designed for
"later addition", and the car was wired for it, but maybe there is some
front-to-back wiring that is needed, for the QC Charging Controller.
I will not get the LEAF without the QC option.
Without the QC, I consider the LEAF a Local-Area car, and that would
be sufficient for many people and their uses. However, I would like to
do "way-point" charging to Range-Enhance, traveling North past LA, East to
Palm Springs (eventually Phoeix), and South to San Diego.
When the QC stations are along I-5, I will be able to drive to Oregon, and
to visit friends in Seattle. Of course, the fees to use the QC stations might
be way too high to be economical, but ... I hope not, in interest of facilitating
the transition to EV transportation.
See SOC/GID-Meter and CAN-Do Info
2011 LEAF, 2014 Tesla S85
2018 & 2019 Tesla Model 3
Solar PV: 33 x 225W -> 7 kW max AC
Craigslist: Xm5000Li Electric Motorcycle