Tes, you can.
Tutorial: You have to start a career, choose a number and a team. Then you have to go to the game folder and open "moto3.mix". Now you have to take the livery (two files), the tires, the glass and the chain, I suggest you copy them on the desktop. Now you have to change the name (es Bike999_lod or Rim999) the numbers must be the same and not present in the game. Once you do this, put the number on the motorcycle livery and put all the files back into the folder. Now when you replace the old file (Moto3.mix with the modified one) when you go to play your rider you should have those changes. If there is something that is not clear to you, write me (Sorry for my English but I don't speak it very well)
Tutorial: You have to start a career, choose a number and a team. Then you have to go to the game folder and open "moto3.mix". Now you have to take the livery (two files), the tires, the glass and the chain, I suggest you copy them on the desktop. Now you have to change the name (es Bike999_lod or Rim999) the numbers must be the same and not present in the game. Once you do this, put the number on the motorcycle livery and put all the files back into the folder. Now when you replace the old file (Moto3.mix with the modified one) when you go to play your rider you should have those changes. If there is something that is not clear to you, write me (Sorry for my English but I don't speak it very well)