To do it properly, you'll need a spring compressor. Space is tight in the strut tower under the wheel well.
I am imagining a scenario where, while the car is on the ground, you unbolt the nut that attaches the strut to the strut mount, then slowly jack the car to release pressure on the spring. While I won't comment on the safety of such a procedure, it just might work. I'm not recommending it though.
At that point I'm not sure the spring has enough room while uncompressed to get out of the strut tower. Since you're trying to do something that is not recommended procedure, it's going to be trial and error at your own risk.
I will say that, even with spring compressors and doing it properly, getting the spring strut assembly off of the car by yourself is a major pain.
2009 M3 Sedan DCT - Alpine White / Anthracite Speed Cloth
Mods: Fabspeed X-Pipe, Black grills and gills.
Tracks driven (in any car): Nürburgring Nordschleife (05/2011), Eagles Canyon Raceway, MotorSport Ranch Cresson, COTA Austin