::.. Retrofit Microfilter in 'older' cars

[Qube]

Is it possible to retrofit a cabin microfilter in the older cars that don't have them? Would it just be bolt on parts or is extensive modification necessary?

[shogun]

Has beem discussed many times on other boards, also on the German E32 board. Apparently it is not possible unless one installs a new complete IHKA unit. But maybe someone knows better. A lot of people with cars w/o microfilter would like to install one, as pollen allergy is a big issue in some countries. Also here in Japan, as we have many pine trees. Sometimes during the season it looks like I am driving through a sandstorm, all red/yellow from the pollen.
Here is a unit w/o (at least I assume so) microfilter. Maybe someone has a pic or drawing of a unit with microfilter to see the differences
http://bmwe32.masscom.net/johan/ihka/heater_core.jpg