Hi, sorry I don't speak french but I can try to help. You'll want to first get the .map for ra3map12. If you only have the .pk3, you can do this by unzipping the .pk3 and getting the .bsp. Then use NetRadiant to convert the .bsp to .map. Once you have the .map for ra3map12, you should be able to follow the steps on page 1 of this forum thread.
If the above doesn't help, contact me on Discord and I can try to convert ra3map12 to Reflex for you. You'll still need to do a lot of work to make the converted version ready to play in Reflex, though.