Gael, You've got two options.
1. Use a spring compressor that slips inside the spring and can grab the coils.
2. Mount the new axle then add the springs using the car's weight.
Surely that 2nd option is easier, except that it won't be done to quickly.
For option #3, you might get lucky and find a dead 108/109/111 handy that you can temporarily mount your new axle on long enough to get the springs in.
-CTH
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