It's not a good idea to do that on that frame, the drop outs are very small.

You should change your axle to 10mm. What freecoaster do you have?

I think the best way to do it is to grind 2mm top and 2 bottom so you don't change the frame angles.
last time i did it was on a suelo simple a couple of months ago, the easiest way to do it in a good and simmetrical way is to get a 12mm screw, you start grinding the top (or the bottom doesn't matter) until you can insert the 12mm screw, then you grind the other side till you can put the 14mm axle in.
You can use a angle grinder or a dremel, if you want to be very precise when you are close to the measure you can use a rasp for the last part, an important thing is to cool down the metal while you grind it, a good way to do it is to make a 1/2mm hole in the cap of a plastic bottle fill the bottle with water and pour it over the metal.

i have the éclat blind....

You can get a female axle and 10mm bolts for that hub.