it is possible to retrofit anything if you want but you've got to be pretty competant with electronics to do so. There are various relays, the control goes through the ECU. Parts needed would be a new heaterbox, ac rad, all the pipework, assorted bits of wiring and plugs, compressor and bracket, aux belt, different aux tensioner (probably)
Not simple
This^ and a 1.2's acceleration with the air con on is comical, I know from experience.
Does the AC on actually make them slower lol!