You can try, but I would advise against it if you value your life!
Some people believe that a 40mm grenade is a 40mm grenade, end of story. In reality, the MK19 fires linked 40mm which have a slightly longer cartridge. If you try to fire one out of the M203, the breach of the weapon will not close properly. It will likely jam. If it does close properly, I hope I am not around when you fire it.
Consider this: The maximum range for the M203 is 400 meters. The maximum range for for the MK19 is 2,212 meters. The MK19 launches a projective 5 times father. The round clearly has more propellant. Additionally, the M203 is made of aluminum and the Mk 19 of steel.